This page of reviews of electric guitar amplifiers will run heavily to those of two manufacturers, Fender and Marshall. There's nothing wrong with amplifiers made by other companies, these just happen to be my favorites. I like other amps just fine, and do use others for certain things in some recording work; I also love the sound of some other amps in the music of other artists, such as Vox amps (especially the AC30/6), Ampegs, Dumbles, etc. I just happen to prefer these two lines personally, and so I have the most experience with them. On this page I will review some older amps, such as Super Leads ("Plexis"), blackface and silverface Fenders, as well as reissues of older amps (like the Fender Twin Reverb and Marshall JTM-45 reissues and Victoria's copies of "tweed" Fenders), as well as some newer amplifiers such as Marshall's JCM 900 and JCM 2000 lines.
To the left is a photo of a Marshall 4100 Dual Reverb/JCM 900 amp head, sitting on a 1960A slant-face 4x12 cabinet. This is a fairly modern amp design, with a "clean" channel and an "overdrive" channel. The speaker cabinet is much sturdier than the older, classic '60s and '70s Marshall 4x12s, and is loaded with the modern "silverback" Celestions. These speakers are very quick and responsive, and are quite good, but some people prefer the classic "greenbacks" or the modern reissue "greenbacks". The older type speakers sound more "familiar" since they were used in many classic recordings of the late '60s and '70s, and seem a little slower and fatter in sound and response. They also are rated for 25 watts each, and these at 75, so with a 100 watt amp head you have to really push these to get the effect of the speakers being really pushed hard. On the other hand, the "greenbacks" used to have a tendency to blow out when pushed really hard with 100 watt Marshall heads, since Marshall's version of 100 watts tended to actually be well over that!
Frankly, in my opinion these speakers are fine, as are the "greenback" resissues and Celestion's "Vintage 30" speakers. They each have a somewhat different sound, and are good for different tones and sonic textures. These simply have what to me sounds like a more extended range, good for a wider range of musical styles. As for the amp head itself, my personal opinion of the JCM 900s is somewhat mixed. The "Dual Super Lead" version I just don't care for that much; it has plenty of gain, and may be a good choice of amp if you play a somewhat '90s metal or thrash metal kind of music. The Dual Reverb is an amp I like much more, but again my feelings are mixed about it. This particular amp sounded very good, but I think it may have been due to the transformers almost being burned out due to an accident; I've certainly never heard another JCM 900 that sounded as good.
The amp sounded thinner than I expected a 100 watt Marshall to when I first got it, but slowly it began to sound better and better until it was great- the clean channel was clean and clear but crunched like a JTM-45 when pushed, the overdrive channel was thick and loud. It eventually became a screamin' amp- silly me, I thought I was getting more used to using the amp, but then it blew a tube and the fuse. It turned out that it was probably dropped when shipped to me, and the amp had essentially been running too hot all along, and getting hotter as time went by. The amp tech who ended up with it said he was really surprised the trannies took it, but they checked out OK. I replaced the Sovtek 5881 power tubes with Svetlana 6L6GCs, and then it really did sound good. The clean channel was better than the best it had sounded before, and the overdrive was clear but still very gainy. Basically, a modern type amp that sounded good instead of just too much gain and no tone. Still, biased so it wouldn't burn up it never sounded as blazing hot as just before it blew the tube the first time. I was disappointed that I could never seem to help anyone else get their JCM 900s to sound this good, and I suspect that the overheated OT just somehow really affected the sound in a positive way. I also suspected that the PT and OT might simply fail at some point due to that overheating (took about 9 months to go through the above story- that's one of the reasons the amp tech was so surprised the trannies lasted), so I traded it off on a 1970-made Super Tremelo.
The Super Tremelo I have is electronically a Super Lead, since the tremelo is disarmed. The '70 (and early '70s) Super Leads are the same electronically as the 1969 "plaxi" Super Lead, the only difference is the aluminum front panel instead of plexiglass. Some people feel the '67-'68 plexis sound better than the '69 and early '70s SLs, but I disagree (and so do most of the better amp techs I know). There's not a lot of difference, though, a few component values. The 100 watt heads do have some electronic differences to the 50 watt version, though, and I feel the 100 watt version sounds better. They can be easily modified to 50 watts, though, with no change in sound. This will still overdrive the speakers if you use "greenbacks", but won't so much overdrive the more modern Celestion speakers. This particular amp is a somewhat rare model of the Super Trem, a factory built 100-watt in a 50-watt chasis, which makes it much less bulky than usual.
The Super Lead is essentially a 100 watt British component version of the classic Fender Bassman circuit, with a solid state rectifier for more clean headroom and higher gain when it is pushed. If you jump the bright channel into the normal channel these amps have a LOT of gain, enough to really surprise a lot of users of modern high-gain amps when they first experience these classics. 
Being based on the Bassman circuit, the Super Lead is just a great sounding amp- pretty much any guitar sounds good through them. There's certainly more bottom end then the earlier Marshall JTM-45s or the Fender Bassman, both of which are pretty bright sounding in comparison. Marshall's switch to EL-34 tubes in the power amp was a stroke of genius, they have as much tone and bottom end as 6L6/5881 tubes, more bottom end and a fuller, more rounded tone than the KT-66 tubes the JTM-45s were built around, and more gain than either. The use of four 12 inch speakers in a closed back cabinet added even more bottom end to the sound, and created the classic Marshall sound that Hendrix really broke new ground with. The later Super Leads (post '74) switched to PC board construction instead of point-to-point wiring, but contrary to some opinions I believe good quality PCB construction does not degrade sound quality.
The JTM-45 was the early classic Marshall amp, a 45-watt amp head with a tube rectifier. Essentially a sturdier head version of the Fender Basssman, with a few component value changes, and built around readily available British parts, this amp has the classic "crunch" sound associated with Marshalls when overdriven. The reissues Marshall currently makes are excellent amplifiers, and very close to the originals. They employ PCB instead of p-t-p construction, but as noted above, if the quality of the board is good I feel the difference is irrelevant. There are a few relevant differences, though, but as for component values I believe there are only two caps and one resistor that are different from the original 1964-'65 versions. The reissues come stock with 5881 power tubes, which are pretty direct replacements for the original KT-66s, and more available today. In the '60s, KT (kinkless triode) tubes were easily available in the UK.
The tonal differences between 5881 tubes and KT-66s are that the 5881s don't have quite so much "crunch" sound when overdriven, but in my opinion somewhat make up for this by by overdriving at a lower volume, having more bottom end and a little more "musical" overdriven sound. This is one of those "what do you want to sound like" issues, rather than what is exactly "correct".
The one really important difference in the reissues and the original JTM-45s is the output transformer. The new ones are more consistent probably, but don't sound as good to many people (including myself). Hammond and Mercury Magnetics both make very acceptable replacements, but the best on the market today (both in sound and durability) is probably the replacement OT made by Obsolete Electronics.
The reissue 50 and 100 watt Marshall Super Leads are pretty good sounding amps, if you're not used to the originals. Unlike the reissue JTM-45, the "Plexi" reissues just don't much like the originals, and to make them do so seems to require extensive work, including the conversion to point-to-point electronics. Too much work, in my humble opinion, when you can probably pick up an early '70s Super Lead used for around $700 US.
I have recently spent some time using a Double Super Lead version of the new JCM 2000. Frankly, I don't care for the channel voicing of the Triple Super Leads, but I really like some aspects of the DSL. What I don't like (aside from the inside looking like a nightmare to ever work on) is that a lot of the settings of the amp are unusable. There are two channels, each with two "modes". The Classic Gain channel has a "Clean" and a "Crunch" mode. The clean is too clean- with 6L6 tubes it would probably sound good, but with EL-34s it's just too clean, essentially unusable to me. Thin, and just not very pleasing. The "Crunch" mode on the other hand is excellent, one of the best modern amps on the market in my opinion. It has a sound that is somewhat reminiscent of the old Marshalls but has a brighter, edgier and more modern sound without the somewhat annoying harshness or thin sound I hear with Mesa/Boogies, Soldanos, etc. It doesn't sound like a Super Lead, but there's a certain "Marshallesque" quality about it that differentiates it from other modern high-gain amp designs.
The Ultra Gain Channel again has two modes, "Lead 1" and "Lead 2". The first is like the "Crunch" mode of the previous channel with more gain, and is a very nice sound compared to other modern high-gain amps. The "Lead 2", however, is just too gainy and has too much hiss. Noisy and fairly useless, to me. Others may like it. It certainly still sounds better to me than a Mesa or Soldano, etc. This amp also has two other interesting features, a "Deep" switch and a "Jazz" switch, obviously meant to emulate the similar switching settings on Dumble amps. The "Deep" switch scoops the midrange, and the "Jazz" setting somewhat attenuates the sound, but in a pleasing way. In summation, this is an amp with some good features, and some useless ones, but I find it much more pleasing in sound than the other modern high-gain amps on the market. Better tubes (like Svetlana EL-34s) would be an improvement, and if the last two preamp tubes of the five Sovtek 12ax7s in it were something a little less gainy and noisy (like 12au7s, maybe) the highest gain mode would probably be smoother sounding and more musical, not to mention less noisy.
Fender amps are the basis of most modern amp design, the majority of which seem to be derived from the early tweed Deluxe or the late '50s tweed Bassman amps. It would take a lot more time than I have to give any sort of history of Fender amps and reviews of them, so I'll just restrict myself to a few summaries. The reissue '59 Bassman is a fine sounding amp, but not as good as the original (the tone stack is quite different, for one thing). On the other hand, Victoria Amp Company makes exact and exacting hand-made copies of tweed Fenders, most notably the '55-'60 Deluxe and '59-'60 Bassman. These may simply be "the Holy Grail" of amplifiers, and are exactly like the originals, even down to hand-made parts which are no longer available except in these amps (and are all loaded with NOS tubes). If you've heard and love these old amps, the Victorias are what you want. The Bassman copy is pretty expensive, but the Deluxe copy costs about what a used one plus retubing would run you.
The Fender reissue '65 Twin Reverb and Deluxe Reverb are also very fine sounding amps. The Deluxe Reverb is nice, but certainly in my opinion isn't up to the standards of the real "Blackface" DR. The reissue Twin, on the other hand, is (as near as I can remember) exactly the sound of the BF Twin Reverb when they were new. When I was young, and dinosaurs ruled the earth...
"Silverface" Fenders, made from 1968 through the '70s, are readily available and relatively inexpensive. They are also easily modified to Blackface specs, and to adjust to various other configurations (such as Dumble clones). Many early SF Fenders are electronically the same as Blackface models, especially the Deluxe Reverbs. Like I say, these are readily available, cheap, and easy to fix up however you like. They are also not so valued as "vintage" amps that it seriously affects the resale value to modify them. The weakest point with these amps are the speakers, as Fender bought speakers from 3 or 4 different companies during the 1970s, and many of these amps have very inferior speakers. You'll occasionally come across one with good original speakers, though. Brown or white tolex covered Fender amps from the early '60s are usually reasonably priced also, though there are simply not as many of them around. These have those classic "surf" kinds of tones, as the Brown Showman amp head and cab and an outboard reveb unit was the typical Dick Dale setup. The white tolex covered amps were the higher end models, such as the Bassman and Bandmaster.
Personally, my favorite Fender amplifiers are the tweed Deluxe and Bassman, and the BF Deluxe Reverb and Twin Reverb. The Super Reverb was an excellent amp also, I just prefer 12" speakers to 10", although in the case of the tweed Bassman combo you can hardly argue with perfection.