@jwlaff Here are some things to look for to get good bass out of a tube amp:
1) if the amp has output transformers, look for low frequency bandwidth to 5Hz or less. OTLs can go down lower; what's important here is to have no phase shift at 20Hz and above- that's a requirement for good impact. To do that the amp has to be full power to 2Hz. But this might not be that important if your speakers don't also go to 20Hz. Amps that are only specced to 20Hz might not be so nice in the bass. If they really start to roll at 20Hz you can get phase shift artifact right up to 200Hz.
2) Its a really good idea to avoid loudspeakers that are 4 ohms, and especially speakers that are 4 ohms in the bass while being 8 ohms in the mids and highs. So this will included a lot of speakers with dual woofer arrays. On the 4 ohm tap most output transformers are less efficient (more of the amplifier power is being converted to heat in the output transformer) so you get a little less power, and quite often less low frequency bandwidth as opposed to the 8 or 16 ohm tap (the latter being the best; 16 ohms is the way to go with tubes). IOW, to get the most out of your amplifier dollar, avoid low impedances (this applies to all amplifiers, not just turbes).
3) Most speakers (but not all by any means) are meant to be 'voltage driven'. This means to get flat frequency response the amp has to behave as a voltage source i.e. it can put out the same voltage into any load. No tube amp can really do that (neither can solid state) but they can be quite effective at this even with 4 ohm loads. What is needed in a tube amp to do this is a fair amount of negative feedback. The problem with this is that feedback introduces distortion of its own (while otherwise suppressing distortion and reducing output impedance). This distortion is audible as brightness and harshness; these qualities are not why people buy tube amps- they want smoothness and detail instead.
To get that many tube amp designers use other means to obtain linearity (triodes, class A, etc.) thus allowing them to run the amp without any feedback at all. This works really well, but the amp will not behave as a voltage source. Despite that, you can still get very neutral response but you do have to be more careful about speaker/amplifier compatibility. In high end audio, this is a well-known problem regardless, so this is an easy thing to do- the manufacturer of the amplifier can give you some good suggestions. For more on this issue see:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.phpIf you do this right you can get outstanding bass. I have yet to hear a solid state amp that can keep up with the bass I get at home; keeping in mind as FWIW sort of thing that I've played string bass in orchestras and bands since junior high (not that such really qualifies me as a bass expert, just that in my audio system I have to get the bass right to be satisfied). But my amplifiers have full power right down to 2Hz and my speakers are 16 ohms in the bass, go to 20Hz and are meant for tube amps.
Another way of putting this is quite simply regardless of the amplifier you choose (tube, solid state or class D) you really don't want that amplifier to be working hard (it will make more distortion and be less musical) so there's no point in trying to make a tube amp drive a difficult load. You'll get substandard reproduction and you'll go through tubes faster. Solid state amps might be able to drive low impedances better, but they too make more distortion in doing so, and the whole point of high end audio is to make the music sound as real as possible. So make sure your amp is driving an easy load (**especially in the bass**) and you will be rewarded.
