I agree that placement, integration and a properly treated room make all the difference in subwoofer impact. If you haven’t already, your best $ spent is on a Umik1 and downloading REW to get a baseline of what your room is doing, the in-room response of your speakers without subs, and then bump in your subs and integrate from there. As previously stated, you likely have a phase issue making the subs cancel out the speakers. That could be just at your listening position, but possibly across the whole room. Spend most of your attention here to get best results with whatever sub used.
All that being said, I recently went from using two SB2000 which were very well integrated over to a single GR Research sealed sub. I did lose the tiniest bit of stereo separation (higher crossover point and small listening room) but the sub is SO much more tactful punch, articulate and musical. The notes come through worlds better and the punch of a kick drum has snap to it instead of a thud, especially through heavy repeated hit passages like double bass metal drumming. Having now heard it, servo subs are so much more controlled and punchy that I can’t go back. Also having a PEQ built into the plate amp really helps control any room modes and integrate EASILY into your system. If you watch the used market, they do come up for a real bargain from time to time.
Just my $0.02.
-Lloyd