I live in a NYC apartment, and my living room is about 12' X 21', but it's open another few feet. The PB12 is more than enough for my room. I've never felt it lacking for music or movies.
But, people who buy SVS subs don't normally stop at "perfectly adequate". So, would the PB12-ISD be more than enough for your room? I don't doubt it. (There are others here who own the PB12 also, so maybe they'll chime in). But again, if you will regret not buying a bigger sub to begin with, I'd say go for the ISD/2. It seems to be the sweet spot in the SVS lineup in terms of bang for your buck.
And I wouldn't say that Yamaha and Denon have passed H/K. Yamaha has its backers, but I'd say H/K and Denon are generally viewed on a bit higher plane. And then people just have their favorites. You can't really go wrong with Yamaha, H/K, or Denon, but I have read that Yamaha's may not be the best match for Axioms.
