If the sub is powered, the safest bet is to set the front speakers to small, the crossover to 80, and the EQ to 2.5 KHz. This may not give the best sound, which will depend alot on how your room is set-up, but is a good starting point. See how that sounds, then tweak a little with the crossover.
Make sure the crossover on the sub is set as high as it will go. You want the crossover in the receiver to do the work, not the one on the sub.
If you don't have one of the test disks, you can use the THX tests off the Star Wars Trilogy or a few other DVD's to evaluate the quality of the sound.