Tubes are inherently lower distortion than solid state devices; particularly in the case of triodes. Not only are the characteristic curves of tubes more linear than those of transistors, but crossover distortion is virtually eliminated. It is possible to build an excellent tube amplifier without negative feedback to reduce distortion; this is not generally the case with solid state devices. Solid state devices are electrically much more fragile than tubes and a great deal of the circuitry in a solid state amplifier has to do with preventing self - destruction.
The supply of tubes and tube type components is better now than it has been for many years. Many high end audio manufacturers are making tube type equipment, and there are several people making very good quality kits. I have added several links below that may be helpful in this regard.

My home system consists of two homebuilt Dynaco^® modified Williamson amplifiers, a Dynaco^® PAS 3 preamplifierwhich has been rebuilt with a kit available from The Parts Connection^® , and a Harmon - Kardon ^® F500X stereo tuner (plus some - ahem - solid state devices which I won't mention). Click the soldering iron to see the schematic for the Williamson amps. All the parts to build these are readily available from the suppliers below (don't ask how much it all cost). I used Hammond transformers, and Svetlana EL34's in the homebuilt amplifiers. Most of the small parts came from The Parts Connection, the tubes from The Tubestore, and some parts from Welbourne Labs. Other parts were from Antique Electronic Supply. I found all these suppliers to be reliable and their products were supplied as ordered.