I missed that... you are getting overheating faults? That is definitely a bad sign.
As mentioned above, the Polk Speakers are not friendly electrical loads. You could be well on the way to blowing out your AVR.
Do you employ an active cooling solution on your AVR? Something like the AC Infinity Aircom units:
This quiet fan system is designed to provide efficient cooling for receivers, amplifiers, DVRs and more.
www.acinfinity.com
Or more simply just some 120mm fans:
Direct exhaust air effectively utilizing the Multifan S2. This quiet 120mm USB centrifugal cooling fan pushes hot air safely away from home electronics.
www.acinfinity.com
Protect your electronics cabinet utilizing the Multifan S7. This quiet dual 120mm USB fan system delivers superior airflow to maintain optimal temperatures.
www.acinfinity.com
(Just examples, but I use an Aircom similar to the first link and it is great!)
Anyway, the other aspect of this is that you may well be overdriving the AVR with multiple Polk Speakers. Getting good amplification that can handle the current loads posed by those speakers is important. Bi-Amping will not help that, though, just to be clear.
The electrical load of those speakers will generate a significant amount of heat in the Amp Stage which then must be dissipated. Heat is the number one killer of electronics.