many tube amps are closed in on top, not airy. so they may be butter
smooth, but it’s a circuit artifact overlayed over all the music, and
not ’the’ music.
@mikelavigne The quality of smoothness is a lack of higher ordered harmonics; also a lack of IMD. 'Airy'-ness is arguably a different quality which probably needs more clarity to make sure we're talking about the same thing.
In my book, 'airy' has to do with the speed of the amp; many tube amps are not all that fast (15Volts/usec is common). The other thing that this might refer to is phase shift. If there is a roll off in the amp that isn't all that far from the audio band, for example 50KHz, this will introduce phase shift down to 1/10th the cutoff frequency. The ear interprets this sort of thing as a coloration (since it covers a spectrum rather than a single frequency); if the phase shift is due to roll off, it will interpret is as 'dark' or 'slow'.
Not all tube amps behave as you describe of course (hence your use of the word 'many' rather than all). If the amp has bandwidth to 100KHz then its going to have minimal phase shift and won't sound dark. How it generates the higher ordered harmonics is a different matter, but there are tube amps that are both smooth (lacking audible higher ordered harmonics) and 'airy' at the same time.
