Perhaps you could build side tables for your couch. If you just made wood supports on the corners, and packed the whole inside with insulation, I think you could make some very effective traps. Since you are trying to suck out very low frequencies - this could work really well. Just an idea. You could also just pull your couch out a couple inches from the wall (and if your couch is actually sucking up some bass, this might make it even more effective...or less, who knows) and make floor-to-ceiling 6" deep traps behind the couch and to the sides of the couch on the other side of the angle. I'm sure you can see how corner traps are so effective - a 90 degree angle can cause some serious resonating, but I can't see a 135 degree wall being nearly as bad as a 90 degree. I'm no expert, but it makes sense. Perhaps another idea would be to make some standard bass traps, and just not have them go floor to ceiling. Mount them in the 135 degree corners above the couch, and just have the angle behind them in the middle, if that makes sense.
I don't know. Just ideas. Pros can let you know whether any of these are actually good ideas..
Anyways, good luck man.