I think this should be the next course of action as well. Hopefully you have a friend with an amp that you could somewhat easily get to. Find a long set of speaker wire and park right by each other. Run his speakers or sub from your amp, and your subs from his amp.

I've had an old Rockford Fosgate amp give out a few years back, and it gave that constant clipping to the sub I had it attached to. It ALWAYS sounded bad though, not just high frquencies, so it's not the same problem - could be similar though.

In short, I've seen the amp be the problem before, I'd highly suggest running your sub from a different amp as a test, and ditto your amp to another sub.