I just noticed today that when one of my subs powers up it sounds as though I can here air from the woofer itself. It seems to be holding its own in output. I checked it with the meter and it matches the other sub. They are stacked. I have to put my ear up to it to here the sound but it most likely will get worse? The gain knob has no effect on the mentioned sound. Guess I will have to pull amp and send it back. Anyone have this problem before?