HI Vince
Firstly on sub positioning in your room I agree you are going to get the best response in that corner. Your room is large and leaky so getting maximum room gain is what I would strive for. Just bear in mind that the tradeoff will be more peaks and nulls so strive for a sub with great linearity and be prepared to move the sub around in the corner or move the couch forward or backward somewhat to find the best listening position.
As for choice of subs any one will do the job for you so the question is which one will work best. The SVS sub with dual drivers will likely be the loudest but FR is not as linear in SVS subs which will accentuate your problem with room nodes. Also the drivers in the PB12/2 are not as good as the TV-12 in the Ultra I currently own so the sound quality will be compromised somewhat. In comparison the sound quality of the Axiom trumped the SVS Ultra hands down in the recent listening test which I participated in. All things considered I would back up the advice you have received and favour the 500 (or ideally a 600 in your leaky case) over the SVS alternative.
Hsu makes great subs too but not having heard it I have to reserve comment. I did however have a long talk with Dr. Hsu last month about his design philosophy which is remarkably similiar to Ian's. Nevertheless, I would surmise that the VTF-3 can't be as linear as the 500 because it doesn't have a DSP chip controlling the FR.
My suggestion is to order the 500 and test it for 30 days. I've checked out many subs recently and spent countless hours talking to Ron/Eric at SVS, Dr. Hsu and of course the good folks at Axiom before ordering 2 600s and 2 500s for my HT. While they are all pretty good at that price point Axiom subs are a cut above the others.
