What is the reason for tubes not as popular as solid state amps?
First of all the vacuum tube market is small and what they call NOS tubes ( New Old Stock) are ridiculously priced just because some people crave for them and so the seller responds with higher prices !
Tubes produced today are quite good. However they aren't exactly cheap !
For power amps you need output transformers too except for some designs and good ones are quite ( very !) expensive. Chassis are heavier as they have to take all that weight from the transformers and chokes ( if any) . Takes longer to assemble too. Hence production runs are comparatively small. So they generally start off at pretty high prices. And in a small ( tube) market where people go after tube amps only if they want them so badly , the prices are expected to be bumped up higher.
You get tubes with the same type number from old stock or new stock from different manufactures. They have the same number and are generically the same but NOT exactly the same. So they could tend to sound different when you switch tubes types. The mind plays a huge part here. What you prefer someone else does not ! If many people prefer a particular brand , the price skyrockets , for NOS tubes. Typical supply and demand effect.
Generally speaking tubes audio amps have poorer bandwidth ( that does not mean it doesn't go to at least 20 Khz) and have relatively much higher distortion than equivalent solid state amps. But all distortions are not the same. The ear is more friendly to some types than other types.
Most people think tubes amps sound "warm". That is the result of high second order distortion and it's harmonics. With some exceptions most solid state amps have higher odd order harmonics which sounds harsher to the ear if they are in the audible range. Modern amps do have low levels of high order distortion but that isn't the complete story.
Tube distortion is typically monotonically decreasing...basically decreasing in amplitude as the harmonic frequency rises . In solid state designs this is not always the case. The best of course do have very low distortion over the whole bandwidth but we have seen that this doesn't always result in 'good' sound. Typical Jap amps from the 80's had vanishingly low distortion.But they didn't "sound" as good as amps with much higher distortion. This topic is complex and can't be fully addressed in a post like this.
About SE tube amps. A few things that set them apart from a typical solid state amp.
1. Very high second order harmonic distortion
2. None or very little negative feedback to degrade the "distortion spectrum". And that is audible.
3. High output impedance. Causes a bass lift making the sound even "warmer".
4. Very gentle clipping when over driven. Making them sound typically louder than what it's wattage spec says.
So is it better than a very good solid state amp ? Certainly not specifications wise ! It depends on what your ear likes to hear. If you prefer it , it's for you. But it certainly is not an amp which reproduces without adding anything to the input signal, on the other hand it adds a LOT that was not there! But if you like it , it's for you ! Typical tube preamps without much feedback and buffers have significant second harmonic distortion levels which if not too much, sounds "nice to the ear" ! It's like adding a cream topping to your dessert ! Alters things but you like it !
Might add here that this solid state and tube amp discussion has been going on for many years. They have done several double blind tests over the years. The conclusions were that you cannot hear the difference between a " well designed" no compromise tube amp and the best solid state amps. They WILL be very expensive !
The best way to buy an amp.
1. Don't look at any specs!
2.Check if it provides you the power you want. 20 watts rms per channel is enough with average sensitivity speakers (86 to 88db) for people who like to converse while music is played. 50 watts if you like it loud and you can't converse while playing it loud. 100 watts if you want to blast it so that your neighbors object. Much higher still if you want stomach pounding bass and have a large bungalow so that the neighbours are much farther away and don't complain !
But in today's world, 20 watts or at most 50 watts per channel is good enough.
3.To get the bass power just add a sub ! Overall sound will be VERY loud and clean. Even a 10 watt per channel tube amp with a sub can sound superb ! ( note that you will need a crossover that also filters out bass from the main speakers !)
4. Audition the amp, especially in your home with the rest of your system ! This is very important.