I have a set of 4x 402 bose 2x 602 B&w which are wired into a nad 705 receiver. I'm also using the bose 402 equalizer in the pre in/out. My questions is what kind of subwoofer should i be looking for so the i could hook it up into my receiver. I understand that some subwoofer have built in amps. Should i be avoiding these? I really know nothing about subwoofers. Also if you have any suggestion for subs between $100 and $300 (preferably closer to the middle) it would be much appreciated.