I have the Plus/2 in 8200 cu ft, and when I first got it, it was monstrous, as I have never had a decent sub... I then added a Plus to that room, and together the two are great... I have tons of bass, but I can't really get alot of real low effects as the natural tune of the Plus/2 is 20hz... I have both set at 16hz and still have tons of headroom...
With all that being said, If I were to buy SVS again, and I certainly wouldn't hesitate, my experience with SVS has been like everyone elses - Phenomenal I would certainly be buying Dual 13's... Im finding that the sound quality of the Plus line not as accurate for my taste. Not that its bad bass... just looking for more accuracy...
Instead I decided to go the route of DIY building a couple of 15" ported subs, with some really high quality drivers... I am looking forward to the results.
I think the best idea would be the Ultra 13 for performance and SQ.
By the way, I believe SVS has stated that they have roughly about the same output, with the edge going to the Plus/2 above 30hz, but the 13 is able to reach really low... and should be more then you need, but dual subs are where its at...
Good luck