For one thing, the Dayton amps make far less power than they claim, and it's only 300 watts at 4 Ohms claimed. I expect actual power at 4 Ohms is around 200 watts at the most. Wired to 8 Ohm.... 125-150 watts.
That is woefully insufficient power for the Kappa Perfect 12" VQ.
You are using the avaserfi cabinet, slot ported, right?
A pair of the Kappa Perfect VQs cabinets as provided by avaserfi, powered by an Ep2500 amplifier will produce a very substantial amount of output down to 17-18Hz. Now, with that HUGE volume you have to filll with bass (the room is pretty good size - but with huge openings to the rest of the house - this makes it a most difficult situation). As far as quantifying the difference; if you will settle for relative comparison, here it is: the proper amp will give you about 60% more output, distortion free, from the subwoofer. If you add a 2nd subwoofer, then this will result in roughly 300% to 400% more LF output as compared to your current situation. It will have no audible distortion or compression, presuming correct set up and build, and assuming they are next to each other/close. I highly recommend a subsonic filter set at 20Hz to protect the drivers from over-excursion under the port tuning frequency if a movie LF effect happens to occur under the port tune frequency at high SPL level.
If you want substantially more output capability, and with the same high SQ, you have increase the budget considerably.
-Chris