Hmm. Well, the Axiom EP600 toughts that it can handle most any room size. So it sounds, from a marketing standpoint, to accomodate your room size. To say it's the all in wonder sub, well, we need more people to be getting them, trying them, and posting on it to know for sure. That is Axiom's problem at this point in time. It's too new to tell. What we need is at the very least some magazine or web reviews on it. And moreover, a brave and independantly wealthy young soul to do an A/B comparison for us to match the Axiom up against the other sub kings SVS and HSU.
I personally have the SVS PB2-Plus. Somewhat comparable to your stack there. Though two Ultras would have an edge on the plus by a slight margin. Better drivers at least. As far as output they'd probably fair similar as the PB2-plus would be two 12" drivers as well.
To put it in contrast, I have a 15x20 room with 15' vaulted ceilings as well (albeight only till the end of the year as I am moving). But I'd say to some degree we have similar rooms. Mine opens up into other rooms in the house (dining room on one wall, and hallway on the other side). The PB2-plus offers 3 settings. 25 Hz, 20 Hz, and 16 Hz cutoff. The higher the Hz cutoff (25 Hz) the more air it can push in the room. This is the most room filling and it does not disappoint. The lower you go down to 16 Hz, you'll notice a drop in room filling bass, but now you're at the level where your floor and couch are rattling more than you are. This is the stuff that tests your walls integrity. They're all pleasing in their own way. And I'd imagine in a smaller room they'd do even more damage. Though after countless phone calls to SVS they hesitate to suggest a "second" sub unless, when calibrated correctly, I don't get enough output. Well the bottom line is...I like to drive it several decibals past the "correctly calibrated" level...and I have room to spare.
Oh, and BTW. Just because the SVS has a 16 Hz cutoff doesn't mean it doesn't and won't dip below that. They've done tests to prove it can hit the single digits if called upon.
Good luck on your subwoofer quest! Either way I'm sure it'll give you a good kick in the gut. Enjoy
