MichaelB summarized it nicely. Here's my more long-winded take!:
For use in audio equipment, tubes (valves) are obsolete. Period. (I understand that tubes still have a place in some RF and microwave applications). If you were to ask an electrical engineer to design a piece of audio gear from the ground up, (s)he would not reach for the tube catalog! As MichaelB pointed out, they often do add some distortion that some find pleasing. So if you like your music filtered through the audio equivalent of "rose coloured glasses", then valves might be for you.
It is possible to design tube gear without audible distortion, but even then the measured performance would be worse than solid state and the reliability would also be worse. So why bother? Also, our engineer could replicate any "euphonic" characteristics of tube gear with solid-state components if that were desired.
It needs to be said that the designers and makers of "exotic" high-end gear (both valve and solid-state) are often not degreed electrical engineers, but hobbyists-turned-entrepeneurs, or engineers who decided to cash in on a lucrative market, engineering principles be damned!
In this day and age, tube audio equipment is an exercise in nostalgia, marketing, and myth. And an expensive one at that.
If valves still intrigue you, you can try them out for little money by purchasing used "vintage" equipment on eBay, Audiogon, etc., by old (often defunct) makers like Fisher, Scott, and Dynaco (these were US makers; UK equivalents surely existed). The basic technology of those and the expensive piece you mentioned isn't all that different!
(Note: some readers will see that I have cribbed here from Peter Aczel et al. Credit where credit is due...)
Also see the following:
Valve amplifiers - do they really sound different?
And this from Doug Self's article,
Science and Subjectivism in Audio:
The "valve sound" is one phenomenon that may have a real existence; it has been known for a long time that listeners sometimes prefer to have a certain amount of second-harmonic distortion added in, and most valve amplifiers provide just that, due to grave difficulties in providing good linearity with modest feedback factors. While this may well sound nice, hi-fi is supposedly about accuracy, and if the sound is to be thus modified it should be controllable from the front panel by a 'niceness' knob.
