Amfibius, Thank you very much for the explanation. I had ignored damping factor as a possible reason for this "resonance" because Jmaldonado talked about the amplifier rather than the amplifier/speaker interface. I realize that speaker impedance curves are (generally) not linear and for most speakers, will drop around the bass (except for my former ESLs) leading to the lowest DF at the lowest bass. I guess I automatically assume that people, especially those with low-power SETs, would seek appropriately matched speakers so they would get speakers with low DF requirements. My tube amps do quite well with my efficient and high impedance woofers, though my amps are "cursed" with very large OPTs and a surfeit of power and my speakers are designed to use low-power amps.
I wonder if that is the reason why we tube amp owners "miss" the lowest frequencies in the bass... I had thought that the main reason I was missing 12Hz was that my speakers reportedly only go down to 18Hz :^).
In any case, I would have thought that once one gets down to below 40Hz, it becomes much more an issue of the speaker's frequency curve than the amp's (if properly matched) and in the real world, anything below 60Hz and I expect that room loading/acoustics will make a mockery of the "flat frequency response curve" that an amp, tube or SS, produces.
I wonder if that is the reason why we tube amp owners "miss" the lowest frequencies in the bass... I had thought that the main reason I was missing 12Hz was that my speakers reportedly only go down to 18Hz :^).
In any case, I would have thought that once one gets down to below 40Hz, it becomes much more an issue of the speaker's frequency curve than the amp's (if properly matched) and in the real world, anything below 60Hz and I expect that room loading/acoustics will make a mockery of the "flat frequency response curve" that an amp, tube or SS, produces.