fhw,
Are you trying to say that your sub is rumbling enough, constantly enough that the walls are vibrating and hence causing your speakers to vibrate which is causing a buzz?
Do you have an all wood basement (perhaps subfloor) or is it cement block/cement below the carpet/behind the drywall?
I would find it odd that moving to a larger room would actually cause an increase in vibrations over a smaller room unless the rooms were made with different materials.
Have you tried moving your sub somewhere out into the middle or further back in the room, away from any walls to see if vibrations decreased?
You certainly have an odd problem there.
