OP, it is probably the Logic 7 amp. My car has the same problem. You can see my thread here for more information.
http://www.e90post.com/forums/showthread.php?t=319983
In my case, the sound sounds "fuller" and "flatter", but it can sometimes sound like an increase in volume. My sound setting for the fader is 4 clicks to the rear and on that setting, I can really hear the fuller vs. flatter sound, rather than attributing it to volume increase/decreases.