^^ Don, getting good bass is always about matching and bandwidth. One issue frequently leveled at tube amps has been how they present bass, but IME this has a lot to do with how much bandwidth the amplifier has in the octaves below 20Hz. In a nutshell, if you want the bass to be right, there had better not be any phase shift at 20Hz. This requires a cutoff frequency of no more than 2Hz. This is a bit of a trick for transformer coupled amplifiers in general. I know of exactly one transformer that can handle an amplifier that has that sort of bandwidth (the ZERO, for those interested...
http://www.zeroimpedance.com). Of course we make sure we have that sort of bandwidth
at full power in our amps.
Matching is another issue, and no doubt controversial. Here is why: In high end audio, there really is no good reason for low impedance loudspeaker loads. The reason is simple-
all amplifiers, tube or transistor and including class D, have higher distortion driving 4 ohms and less. Its easy to see in the spec sheets. The distortions involved are often higher ordered harmonics, causing the amplifier to be brighter, harsher and less detailed all at the same time. In addition the speaker cable gets really critical in low impedance applications as it becomes a significant part of the damping factor expressed to the loudspeaker.
High end audio is all about finesse and accuracy; adding distortion does not seem like a viable pathway to that goal at first blush, and upon later examination, is found to be unquestionable
Now if
sound pressure is your goal, there is a weak argument for lower impedances, which is about 3 decibels with some amplifiers. I always find it amusing that certain speakers get a reputation for being difficult loads; folks, a difficult load is never going to sound like real music, plain and simple. Its going to cause the amplifier to make too much distortion.