I have another idea. It might be a bad one but it is something that I'd try, anyway...
You've said that you've tried running with just one speaker and you're still getting the shutdown. I wonder what would happen if you tried bi-amping just that one speaker? I wonder if you'd get a different shutdown point, or if it would in any way work better, or at least give you an idea of which set of cones is causing a problem for the XPA (tweets+mid vs woofers).
Put an RCA splitter on one channel output from the prepro, and run the splits into the inputs of the XPA, left and right. Then remove the bridge plate on the M80 and run a separate wire from each terminal on the M80 to each output on the amp. Then try it again. I'd be curious to see if it shuts down at (about) the same loudness.
