Having made the argument for Vacuum Tube guitar amplifier superiority, in this paper, I will explain when solid state amplification is the way to go.
Solid state amplifiers are excellent at amplifying sound that is converted to electrical signals. As I expressed in "Why Vacuum Tubes", their tonal purity itself makes solid state amplifiers not the best choice for guitar amplifiers. However, in applications where amplification with no coloration is desired, Solid State is the best choice. Solid state amplifiers can be near perfect at sound amplification.
Vacuum Tube amplifiers have limitations. The first limitation is scalability. Building a Vacuum Tube amplifier beyond 100 watts of power stretches the limits of tube designs. It can certainly be done but it requires a lot of power output tubes and weighs a ton. It gets expensive real quickly. In the early days of rock music, it was common to see a wall of Marshall or other brand speaker stacks lining the stage. Although partly for show, many of them were active and each was powered by a 100W tube amplifier. This was necessary to fill the venue with the loud rock music and be heard over the crowd noise. Today, a 20W tube amplifier may be used and achieve the same volume. A microphone is placed in front of the tube amplifier and the solid state arena sound system amplifies it and easily fills the arena. This is the best of both worlds, tube guitar amplifier and arena filling sound accurately amplified by the powerful solid state amplifiers in the equipment rack of the sound system. Solid state amplifiers of 1000W or more are small, lightweight and waste no power in the vacuum tubes heaters.
Vacuum tube amplifiers are AC coupled and have limited low frequency response. This is because the output transformer's response rolls off at lower frequencies. It takes a lot of iron in the transformer core to go down to low lower limits of the audio spectrum. The lowest frequency typically played with a guitar is the sixth open E string. With standard tuning, that is 82.4hZ. Crossfire Electronics amplifiers extend their low frequency response this low by using the best transformers available. These transformers are expensive, large and contain a lot of iron. Most tube type guitar amplifiers do not do this. Solid state amplifiers however are DC coupled and can have frequency response arbitrarily low by the nature of their design. This makes solid state a natural choice for bass guitar amplifiers. Solid state amplifiers can deliver heart throbbing lows that tube amplifiers would struggle to do. So solid state amplifiers would be a logical choice for a bass amplifier.
Steel guitar sounds are very pure. Steel players do not push their amplifiers to overdrive and musical purity is desired. Distortion is not desired. So solid state amplification would be a logical choice for steel players although country players looking for that 1950's vintage sound may still prefer tube amplifiers but are careful not to push them into distortion. A typical steel guitar solid state amplifier that I see here in the shop is the Peavey Nashville 400. It is a quality, sonically pure amplifier for steel use. It lacks the drastic frequency response curve that is designed into guitar amplifiers.
Keyboard players also share some commonality with both steel players and bass players. They need good low frequency response, flat frequency response curves, and sonic purity. A Peavey KB/A 60 is a typical solid state keyboard amplifier.
Audiophiles still treasure vacuum tube stereo amplifiers. Old Macintosh amplifiers still fetch high dollar in the resale market. Macintosh is still in the business of making vacuum tube stereo amplifiers, fetching thousands of dollars for them. I recently was in a sound room and listened to a high end Macintosh tube type stereo amplifier played through speakers that cost more that my college education did. I was very impressed. The transformers were huge as they would have to be. The world of audiophiles is a little foreign to me. For the pedestrian like me, solid state is clearly the way to go for stereo amplifiers. Their tonal purity, low distortion, perfect frequency response and almost infinite power would seal the deal for me. You can pay an arbitrary amount of money for s stereo amplifier but I'll bet in a controlled environment, you can't tell the difference between one costing a few hundred and one costing a few thousand dollars. My advice for stereo buyers has not changed since the 70s. Put your money in the speakers and buy solid state.