T_BONE: Another question back at you would be... how much tube power (watts vs output impedance) would one need to adequately power an 8ohm 96db woofer? say, the way a 40W solid state amp might...Theoretically, Watts are exactly the same whether they come from a solid-state or a tube amp, so a 40 Watt tube should produce the same acoustic output as a 40 Watt SS amplifier. In practice, it depends on how conservative are both manufacturers on their power specs, but the differences should be negligible even for differences up to tens of percent (for example, a 40 W SS amp vs. a 30-60 W tube amp).
A more relevant factor is: How loud you like your music? With 40 Watts, you can have undistorted peak acoustic levels of about 112 dB at 1 meter with your speaker in anechoic conditions. Adding the other speaker and the room gain, you could easily surpass 120 dB peaks in your seat!! Ironically, at such loud levels a solid-state amplifier with better bass extension might stress your speakers and cause them to produce a phenomenon known as "Doppler distortion" (don't confuse this with another, notorious concept mentioned in the H-thread). In this case, a tube amplifier having weaker bass would produce a cleaner, more enjoyable sound, provoking the logical conclusion that the tube amp sounds "better" than the more accurate solid-state amp. The morals of the story is: If you want powerful low bass from your amplifier, first make sure your speakers can handle it!
Regards,