Super 7XRS MK2 w/ a front-firing DeepOmega8.
It seemed to thin out the impact of the lower end just enough to matter. I expect that the volume of my space is too large for a single DeepOmega8 on its own (~20'x22'x8'), but it's definitely more than adequate when used in conjunction with the Super7's running full-range. When I have some time, I'll try again with the DeepOmega crossed over closer to 50Hz.
I have 2 subs, and two full systems setup in different rooms. So I've been able to try various sub configurations, with a variety of gear -- including 4 different pair of speakers.
I use Omega 7XRS mk2 in a 10 x 11 room with a single 10" sub. I also have a 2nd system in a 14 x 21 room, with a partial cathedral ceiling -- but still a fair bit smaller than yours. Haven't tried my Omegas in the large room yet -- I typically use large floorstanders (one pair has self powered 12" woofers) to properly load this big space. But with my JM Reynaud Twins (2 way monitors) placed in it I much prefer two subs. One definitely helps, but still too light for my taste.
I realize LF adequacy is very subjective and varies from person to person, but I strongly suspect in a room sized like yours you'd really like what a 2nd sub would add. Not only LF support, but a nicely improved open / spacious / dimensional / layered soundstage. These spatial improvements occur when adding the 2nd sub whether (1) high passing mains and filling bottom w/ subs, or (2) running mains full range and supplementing bottom w/ subs.
I high-pass both pair of small speakers at 100 hz (using the high-pass filter internal to my Gallo sub, everything connected high level), and blend in the sub using its low-pass filter to overlap a bit. Relieving the mains of LF duty allows them to play louder with greater ease and finesse, and less distortion. At all volume levels I hear significant improvements in purity, naturalness and transparency.
I've spoken to two other guys who high-pass their Omegas the same way w/ sub (using 100 Hz and 70 Hz), and they also hear the same non-subtle improvements.
Apart from the change in your LF response, I'm surprised you didn't hear this improvement over the entire range. Maybe using a value greater than 50Hz will free up the Omega driver even more and allow it to play with greater ease / finesse?
PS: From your description, it sounds like your mid bass is too light. Not surprising, given your room size. I'd try raising your sub's crossover point......maybe even as high as 150 - 200. If sub plays cleanly enough that high you should get more muscle and fullness without mucking up the Omega sound.