I have a Beolab 3500 in my garage converted into a Radio Shack - it has heating etc but can get cold. I noticed today that when you turn the sound up the sound is distorted. I removed the covers and the drivers look ok - at first I thought they might need refoaming but the surrounds appear to be rubber. The source is ok as I don't get any distortion from other speakers in the system. Any ideas gratefully received?