Yes, first it would go to the sub. The sub would take the low frequency and pass on the frequencies above the sub crossover. No, it should not degrade the main speakers.
Also, the Adcom pre-amp has a second set of line level outputs (probably for a second power amp. Can I use that instead and connect it to the line level input of a powered subwoofer?
You could use that if it is activated at the same time. In another word, if it is passing the signal without switching it in thereby loosing another active output. You certainly could try it and see what happens. Use a left over interconnect and see.
Will a reasonably priced Yamaha subwoofer do the trick?
Thanks for the help. I've never owned a sub-woofer and I'm confused.
Well, that depends on how much you like low frequency reproduction. If you are after something like a theater experience, look for a better sub
SVS or Hsu. If, just to help a bit with music, it should work out. The only think would be is if it turns on as a low signal is present and it uses a mechanical solenoid for switching it on; that could be audible and annoying.