It definitely sounds like a crossover problem. Swap the offending speaker to a different channel and see if the problem swaps to the new channel. If the problem swaps, the issue is likely with the speaker not the amp. It sure sounds like the tweeter is being overdriven with some low frequency stuff. Do the Polks allow you to bi-amp such that you could disconnect the high freq side and test it that way. It would give you a way to try to overdrive just that speaker and see if it breaks up on more than one channel of the amp.