As you know Ravi, the distortion at certain very high volume moments may be due to the speakers, the amp, or both. I recall that you've previously commented that you run all your speakers at"large". Have you experimented with setting them to "small" to see if the distorted moments improve? Lessening the load on the speakers is one of the advantages of the "small" setting.
If it's mostly the amp then it's possible that if the problem is in the narrow power range where the lower power amp is distorting badly yet the higher power amp wouldn't be,that more power would help. If possible, borrow a much higher powered amp to check this .
