The sub should be fed from the preamp (not the amp), thus eliminating any sonic signature from the amp (which would probably be minimal).
Best of both worlds?
If you used a powered sub (such as a Vandersteen 2Wq w/ an external corssover) along with a tube amp, would you effectively have the "best of both worlds" or would your SS sub amp get too much of a low-end sonic signature from the tube amp.