If you have room for it, check out the
Elemental Designs A7-350. That is 1200 exactly and shipping is free. That will destroy you, it is probably the most output you can get for 1200.
You can also get the
SVS PB12 Plus. It's a nicer looking sub but won't be able to generate as much SPL. It will still be a beast though.
I would also recommend the
Epik Phoenix, but Epik isn't taking any orders for awhile, they are backlogged. I would imagine they will start taking orders again after the holiday season.
You can also get the
Rythmik D15SE, which would have as much output as the others, but will make up for it in sound quality. It also looks the nicest of all of them. Plus, it's the least expensive.
Those are what I would consider for 1200, however you could also have dual subs for that much. Two SVS PB12-NSDs, two Hsu Research VTF2.3s, two Outlaw Audio LFM-1 Plus's, or two Elemental Designs A3-300's would also make for a good subwoofer system.