I'm building out a listening room in my basement and, because we have a smallish home, the room will have to do double duty as a recreational space. Thus, the design constraints dictate an overall 20' x 22' space - not a great start for control of standing waves.

The rear wall of the room, however, is not framed directly off of the block walls. There will be a 4' wide hallway runnign parallel to the rear wall of the room to access a stairwell. I'm considering leaving the two rear corners of the room "open" by not sheetrocking an 18" section of wall at the intersection of each corner. Rather, I'd construct frames covered with grill cloth to insert into the openings, pack the wall cavity behind them with fiberglass, leaving the other side of the wall uncovered.

My question: While I don't expect to avoid the need for bass traps entirely, is this a viable option for controlling bass resonance?

If I understand what you're saying correctly, it certainly can't do anything but help.