Tube preamp for techno and other bass-heavy music?

Your question is of interest to me as well.

I 2nd the BAT.

From my experience with BAT, Aesthetix and CAT line stages, the CAT really excels at the frequency extremes. Incredible bass extension and authority.

BAT, either tubed or SS will give you good, solid, bass.

A more general answer - When Joule Electra was first introduced, they offered the LA-75 and LA-100 line stage. The difference was the LA-75 had a SS power supply so was less expensive. The main sonic difference was a stronger, more dominant mid-bass with the 75. Those of us who heard the comparison agreed the LA-75 would appeal more to someone listening to rock, reggae, etc.

So one consideration could be to audition pre/line amps with SS front ends, rather than all tube designs.

Also, you suggest you listen to a "decent amount of material in the 20 Hz - 40 Hz range". Unless there is dominant electronic instrumentation, that might not be likely. The open E string on a bass is 42 Hz. How much do you actually hear below that? Most folks are not aware of the real frequencies of the music we listen to. Have you heard a 60 cycle hum from a ground loop or other problem? That's pretty low and many speakers do not produce much below that, yet sound like they produce strong bass.

Anyway, good luck and happy listening.