did any of your clipping lights come on.. i have the same amp as you, and have never had a problem, however i have 60's wich are not 4ohm speakers.

i would emagine that one of che channels would start clipping before it would turn the channel off, hence the whole reason for the clipping light indicators at the front of the amp.. the plastic "light" across the front of the amp are where the clipping indicators are located.