Quote:
| Room is already treated...6 out of 8 corners are treated...6 corners filled with Auralex cubes...room and full front wall also treated... |
I daresay your room is not, in fact, treated. When you say "room and front wall also treated," what are you using other than the Auralex cubes?
Quote:
| ...for now I need some absorption for the FIRST reflection, right back of the sub... |
From a factual standpoint, you don't. There's no significant first-reflection phase cancellation at 50Hz unless your sub is at least several feet away from the wall. Your problem is the result of room mode resonance, not the wall behind the sub.
Your best bet is probably going to be to follow the generally accepted practices repeated ad infinitum on this board: get rid of the Auralex and replace it with 6" of 705. If you still have a problem after that, you could look at getting into some tuned-trap-style shenanigans.