The purpose of an amplifier is to reproduce a signal as accurately as possible but with a higher "output" amplitude than "input." Thus, if an amplifier is well-designed, it should make no difference whether it uses tubes or transistors, so long as it is operating in it's linear range. In guitar parlance, this is referred to as a "clean" tone. The tubes and transistors serve essentially the same purpose in either case. It is only when the amplifier is operating outside it's linear range that tubes behave differently from transistors, that is when the amplifier is "over-driven." In most areas of music reproduction, this situation is avoided, as it means that the signal will be distorted. The distortion takes the form of "clipping" - the wave-form above a certain amplifier will be chopped off. Tubes do this clipping differently than transistors, producing a "softer," asymmetrical clipping and favouring even-order harmonics over odd-order. This "softer" clipping makes sense, as a tube operates using electrons in a glass tube - there is a lot of space for extra electrons to collect. The clipping produced by tubes is more pleasing to the human ear and at some point, electric guitarist decided that they liked this sound. By contrast, the clipping produced by transistors is "hard." Everything is chopped off equally.
Unfortunately, many guitarists will happily connect solid-state "distortion" pedals to their $4000 tube amplifier, while still claiming that tubes are better. If we are really to take advantage of the tube's superiority, the distortion part should be done by tubes and what happens after that isn't that important, so long as no other component is also over-driven. This suggests that a good set-up would be to use a tube distortion pedal to take advantage of the tube's more forgiving nonlinear behaviour, while using transistors for the power stage to take advantage of the obvious benefits of "solid-state": light-weight, reliability, efficiency and cost.
In addition, the electronics within an amplifier is probably one of the least important components, again, assuming that the amplifier remains within its linear mode. Far more important, it would seem, are the speakers and the speaker cabinet, yet nobody ever talks about these. These are the equivalent of the sound box on an acoustic guitar which of course is lacking in a solid body electric. The sound box is considered extremely important for an acoustic and guitarists will wax at great length about what types of wood are best, how the bracing will effect the sound etc. Quarter sawn solid woods are considered, almost without question, to be best. Yet speaker cabinets are rarely made from anything but plywood. There are other ways to improve the tone colouring of an electric guitar than just the electronics. It is the sound box that adds the majority of tone colouring on an acoustic. Shouldn't we use the speaker cabinet in the same way? These could potentially be built from a variety of solid tone-woods and made with thinner walls at typically over 1 cm thick, the cabinets of most amplifiers are far too stiff to add much tone colouring.
Finally, just about every electric guitar amp is lacking a tweeter. I pointed this out to a salesperson and he stated, somewhat quizzically, that the high range was important for an acoustic, but not so for an electric, which was mostly mids. Huh? They are basically the same instrument except, as I've pointed out, the acoustic is amplified acoustically with it's sound box, whereas the electric is amplified electronically. We should expect the exact same overtones to be coming off the strings. Moreover, most electrics, with their generous cutaways, have a higher high range.