Thanks for the replies so far everyone.
I've decided to go for one, big sub. I don't know how to measure it's response, but I plan on doing that down the road. If I can get a reasonably flat response on the sofa, I'm set. If not, I'll consider duals down the road.
So I'm down to two choices:
1. HSU-VTF15. This looks like a great value in the 1K range.
2. SVS PC13-Plus. Right about the same price.
The performance of the two appear similar, at least on paper. Will the 12" SVS give equal performance to the HSU?
Also, on a pure aesthetic point of view, which sub do you think would look better. I've always thought cylinders looked pretty fugly, but I'm coming around to it considering the other furniture, etc. But then again, a low box wouldn't be too bad. Either way, I'd probably pull them out to somewhere under the fish picture on the left side wall. I've included a few more pics of the corner without my old sub. PIcs are a bit fuzzy and backlight from my iPhone. The room is actually well lit by sunlight.