VOLTAGE! Well, my problem similar stemed from I have MONO blocks originally, and found left was more prominent than right, a portion of this was listening position, but guess what I found later? That one amp was getting a different voltage than the other, the bias was then imbalanced to the 2 different channels, even with the same power cords, and this happened more when I plugged both amps into the same circuit with common ground than plugging them into 2 totally different circuits, I guess the draw on the same circuit caused part of this, or that the PReamp running was also off from its voltage, I don't really know, but it could be electrical in your voltage drop.. You did not really say what you run, but if its tubes, or Dual mono stereo, or Mono blocks it could be this simple.. But room acoustics and Hallways closer to one speaker than the other have caused this in my experience as well. Good luck