Thanks, Steve Eddy, for your detailed explanation. However, my ears tell me easily, that the bass is tighter on both my SS amps, which has less than 1 Ohm output impedance, than through my tube amps headphone output, which has a quite high output impedance.
If it isn't the difference in output impedance making the bass thight on my SS amps and clearly less so on the tube amp, what makes the difference then?
Well, tube amps typically have transformer or capacitor coupled outputs which can potentially result in significant low frequency roll off. Plus it's a bit difficult to make meaningful comparisons between two amps unless you take care to level match them (which can't be done too reliably "by ear"). Listening to one at a bit lower level than the other can result in a perceived reduction of bass due to the way we hear (see equal loudness contour curves).