I agree, that room is huge. The bigger the subwoofer you can afford, the better. Cheap subs aren't going to cut it in a 6500 cubic foot room! To maximize impact, I'd recommend nearfield placement of whatever subwoofer(s) you end up with. I'd check out the Epik Knight and SVS PB12-NSD as well - they might work well for you too (The Knight and A5-350 w/550W amp are my first choices for a room that size, though, considering your budget).
Caliber has larger cabinet, increasing the output, especially in the lowest regions around 20hz..Other choice for the price range would be a7s-450 which I ordered - a sealed 18" with 1300 watts..it's more expensive now though, after the price increase
It's never enough. It's addictive. If you can, get two A7-900.
Panasonic AE-2000, Carada BW 120", Paradigm Studio 40v3 x 2, Paradigm 690v4, Paradigm ADP 470v3 (rears), Micca M-8C x 4, Volt 6 x 2 (SR), SI DS4-18 in 12cuft X 2, Marantz 7702MKII, OPPO 103D, Emotiva XPA-3,
You have a freaking stadium - 25x35..unless you mistyped it.
For that much space you do probably need an a7-900 ($2.2k)
But if you can't spend that much get whatever you can afford and place it right next to your seating, I guess.
Getting things ready for a new theater room build at the new house.
Equipment so far- (8) SI HT18 in mini martys, speakerpower SP2-12000, Denon 4311ci (may replace), MINIDSP 10x10HD, Harmony Elite, Oppo BDP-103D
TBD- Projector, AT Screen, speakers, acoustics, control system