I didn't expect this thread to take off as much as it did. Thank you all who contributed, lots of useful information in this thread.
I should provide more insight about my room and what I'm looking for.
My theatre is in the basement with concrete flooring, covered by carpet. There also an opening at the staircase which is 62" x 88". Its also an odd shape, the last time I did my calculations I believed I came across 2800 cubic feet, somewhere in that ballpark.
I have a pretty flat frequency curve, unfortunately I do not have REW graph to show you all, but my spl meter, test tones and many configuration/equalization has helped me achieve a flat frequencies, from 30 to 120 +/- 2.5db. My Yamaha receiver doesn't allow me to equalize below 31hz, so my 20 hz frequencies are kind of wild lol, but I drop off pretty significantly below 20 hz. I plan to get a minidsp or a better equalizing software soon
Basically I'm looking for more output in the 15 to 20hz region, headroom and overall output.
I should add I'm from Canada, so unfortunately I have to deal with the exchange rate and can only afford one extra sub at this time. I'd love the Cap 218HT, but that damn thing is too deep!
Television: Panasonic TC-P65VT60
Receiver: Yamaha RX-A2010
Speakers: Energy RC-70's, RC-LCR, RC-10 (4), JTR Captivator 1400 (2), Rythmik FV15HP (2)