JH, welcome. The first point is that although you use the term "clipping", the problem that you describe has nothing to do with that; shutting down is an entirely separate issue. If the similar problem four years ago was corrected with a replacement M60(did you learn what the exact problem with it was?), the Pioneer receiver obviously wasn't the culprit.
Now you've added the VP-160 and a new Pioneer and a similar problem has re-appeared. The Axioms have no unusual impedance characteristic that the Pioneers(and other similar receivers)can't handle with ease if the speakers aren't defective. Since the VP-160 caused no problem when it was connected as one of the two mains in stereo, it appears that it's okay.
One of the most common causes of a receiver shutting down isn't some design inadequacy but rather a problem in the external wiring which causes an intermittent short. Although you say that you "re checked" the wiring, did you actually disconnect and very carefully redo the wiring to make sure that there wasn't a loose strand which could make contact, or did you just inspect it. If you haven't done this, it should be done and has corrected many mysterious shut downs in the past. Obviously the new receiver required making new connections which could be causing the issue.
Your equipment is fine and should have no overheating or other problem at the moderately loud levels(e.g., -9.5)which you mention.
Regardless of the present problem, you should make full use of your excellent EP-500 sub by setting all speakers "small" with a crossover of perhaps 80Hz, so that the EP-500 can do what you paid for.
