I would suggest narrowing the problem to a particular piece of hardware. Maybe you've already done so, but I would swap the cables between preamp and power amp, and swap speaker cables or speakers whichever is easiest. I had a similar problem with a pair of electostatics and it turned out to be the crossover in one speaker was bad. Oddly enough it was the speaker that was loudest that was the culprit.
Jim Robinson
Jim Robinson