How easy is it to get to the back of your pre-amp and amp? If you can do it, try switching your mains for your surrounds at the pre-amp and see if the problem is now reversed. Now try the same thing with your speaker wires at the amp, is the problem reversed? What you are trying to do is to narrow down if this is a pre-amp, amp, or cable problem. Doubt very much it is a speaker issue, but if after all this switching the surrounds are still the problem, then it could very well be the speakers. Keep us posted.