Are you sitting in the center of the room? If so, it could be you are in a bass null. Repositioning the SB12 might help, look into "sub-crawling" method of subwoofer placement. It also might be a calibration issue in your AVR. Something else, which I think is a stronger possibility, is that a sealed 12" sub simply isn't going to be adding that much to your two 10" subs. It should be a improvement, and noticeable one at that, but I don't think it would be a huge step up to what you had before. If you wanted that, will will need to upgrade to something a lot more powerful, and probably a lot larger. The ported subs from SVS is what you should have been looking at. Also the ported subs from Outlaw Audio, Hsu Research, Rythmik, and Elemental Designs, to name a few.