Also, @atmasphere - interesting. When I called Klipsch, the man discouraged tube amps. Maybe that's because I prefaced our conversation with the word "affordable". (I'm still outside of year-long complete house remodel.)I imagine you didn't tell him the speakers were from the 1960s, and I'll bet he wasn't even born by then.
What does "clean" sound mean? Why is what a tube amp can do better than clean, or more desirable?
'Clean' means 'not distorted', but to understand what that means the first thing you have to know is how the human ear works!
To that end: the human ear brain system uses higher ordered harmonics to figure out how loud a sound is. In this regard its not using the fundamental tones. If the higher ordered harmonics (5th and above, so for a 100 Hz tone, 500Hz and above) are altered by the electronics, the ear will perceive the sound as louder than it actually is and will also assign a tonality to the sound. Higher ordered harmonics are perceived as 'brightness' (and also 'harshness', particularly in the case of the 7th harmonic).
Low distortion on paper is meaningless unless we also know what it is that makes it 'low'. Its one thing if we can see the lower orders in the harmonic distortion spectrum. But if we are to take how the ear perceives sound into account, the higher orders should really be a lot lot lower than they are currently with all 'low THD' amps. And by that I mean **at least** 2 orders of magnitude!
Just being 'low THD' doesn't cut it. Now it happens that tube amps in general make less of the higher ordered harmonics than transistors do (this is why tubes are still around BTW); so as a result they don't sound as bright and tend to be smoother. (Tubes often make more of the lower ordered harmonics but apparently the ear does not care so much about that- back in the 1960s General Electric did some testing that showed that people would tolerate 30% of a 2nd harmonic in the sound, but objected strenuously if the higher orders were present at levels less than 0.5%.)
The measure of this is easy enough- when you turn it up, the system should not *sound* loud. It should sound relaxed, despite playing a higher volume. Nor should you get any 'sound of horns' (honkiness); it should simply sound relaxed and a lot like real music. If, especially when you crank it, that it gets uncomfortable (brightness and harshness is indeed a coloration) then you will have a good understanding of why tube amps are still around decades on after being declared obsolete.
Again- a good find- people are always on the lookout for earlier Klispch speakers- they have a reputation of being better than the later ones. I don't mean to cause any undo concerns either- this hobby is supposed to be fun after all :) Have Fun!