Last Saturday my wife took the kids to the movies so I had a rare 4 hour window when I could crank my music as loud as I liked (which is pretty loud). While listening at a pretty high level I started to get severe resonant feedback coming from my left speaker. It would start off quietly (barely noticble) then would gradually build up to loud, booming hum until I turned the volume down slightly. Then it would disappear completely...until I slightly raised the volume again.

I've had this setup in my basement now for about 8 months and never heard this before...but granted I've never played it this loud for this extended period of time before either.

I thought I read something once about how the dimensions of your room can cause feedback. Something like if the width, height, and depth are multiples of each other? My listening area is approxiametely 22'deepx15'widex10'high. The speakers occupy the corners of the width portion(nudged between the sidewalls and the entertainment center) and opposite of the speakers is only about 25% closed off (just the staircase leading back upstairs). I have a rather large circular sofa in the listening room that comes within 5 feet of the left speaker on the long end.

My guess is the sofa causing it...but I guess it could be the room dimensions/acoustics as well. Any suggestions? Can room treatment help me?