I agree with Simon. If you take a modern valve amplifier and put it against any good transistor amp (even from 30 years back) in the lab, it will lose in just about every aspect. But, valve amps have the advantage that most of their imperfections actually sound good. While a transistor amp approaching its limits will create odd-numbered harmonics that sound harsh to the ear, valves distort much more softly, starting already at lower level. Instrument amplifier manufacturers have tried long and hard to reproduce this with solid-state technology.
Old germanium transistors have characteristics that resemble valves a little more than modern silicon semiconductors. That is part of the reason why a Beomaster 900 can sound so nice - just keep it far away from the test lab equipment! Some guitar stomp pedals are still being built around those AC family transistors, just for the sound.
For a while in the '70s, some Japanese manufacturers (mostly Sony and Yamaha) built wonderful amps utilizing V-FET transistors. Their characteristics were very similar to valves, but the technology soon gave way to MOSFETs for reasons I don't know too well (possibly reliability issues).
I personally think the valve pre-amp stuff is mostly bogus when used alone. I would like the preamps to do their job without colouring or affecting the sound in any way. If you want to try a valve amp, use it as a power amp with passive speakers, it is enough to get "the sound". Remember also that the valves (especially in the power stage) have a limited life and replacements are relatively costly.
Buried somewhere with my other eternity projects, I have a DIY valve power amp that has been waiting for me to fix a pair of DIY electrostatic panel speakers (not built by me). Maybe I should dig the amp out and try how it would sound with, say, the Pentavoxes.