The AVR will work, just don't go to 100% on the volume control since the "not so high end" amp might clip more easily than a higher end model.
I doubt that the sub will help at all. Hurt? Maybe. Got wire clippers and an 8ohm resistor?
M60s have nice bass.
I think you can squeeze them in there ok. The backs of my 60s are 17" from the wall, and the fronts are just a tad below 32" away from the wall. Side to side, I have the outside edges 21" away from the walls, and the insides are 30" from the walls. That would leave 7' between them for the rest of your stuff. Will something similar to that work? My room is 16'x16', so you'll probably need to adjust a few inches one way or another to adjust for yours...
