I need to replace the original subwoofer that came with the RCA RT2500 theatre in a box system. It was a small box, less than a cubic foot, with dual 5" drivers. My guess is the drivers were each 6 Ohm wired in parallel. The subwoofer amp built into the receiver is rated for 200w into 3 Ohms. I would prefer a single 12" or even 10", but it seems there are none rated at 3 Ohms. I tried my bookshelf speaker rated at 6Ohms, but I really had to crank the volume to get anything out of it. Would it be reasonable to hook a single 4Ohm speaker to the 3Ohm output? Would there be a noticable decrease in output?