Stretch your budget a bit, and get the best sub you can, as you have a medium size room...
B-Stock page on SVS's site has a 16-46PC-Plus for a good price. I think that would be a great sub for that size room...
If you spend just a bit more then your budget, you will be less likely to have upgradeitus down the road...
I bought a Plus/2 for my huge room, and soon after bought a second sub.
Trust us, it will be worth the extra $$
The SB12 plus is a sealed sub, and if your looking for HT use, it only goes down to 25hz, not bad, but not the earth shattering LFE machine you might be looking for...
I agree with the others here... drop the Velodyne and the SVS SB12plus.
HSU makes a great sub that goes plenty deep but is not as pretty as the SVS boxes, but your looking at SVS cylinders, so maybe looks are not that big a deal to you.