For pre-built subs the best $600 sub is the Rythmik LVR-12 right at $600 delivered. If you can spend more then spend more. Two subs almost always work better than one.
Outlaw Audio will be having their top sub on sale soon for $599 shipped. That will be the best value for pre-built subs when available.
Otherwise check SVS, PSA, Reaction Audio, HSU and Rythmik for other choices.
If you are reasonably handy you can always try a DIY project. There are quite a few kits available. All parts included you only need assemble it. You'll get the best value with a DIY sub. There is a dedicated DIY forum here that will be helpful if you go that direction.
As for your setup, I would first pull the speakers off the wall at least 6" or so. I would also angle them slightly so that they are facing the center of the front sofa. If you are seating in that position you would be looking straight down the throat of the horn.
I would also place a wedge of some type under the center so that it points up so that again you would be looking right down the throat of the horn.
And in general there will be issues with bass response for those seats that are right up against the back wall.
Klipsch RF-62II, RC-500, RS-400, SVS PC12+,
Def Tech SC8000
Roku Ultra, Apple TV, Sharp 70" Quattron