John (Jmcgrogan2), note that his reference to the subs appears to indicate that they contain their own 500 watt amplifier. Assuming that is the case, and if the subs are driven by the EAR amps via speaker-level inputs, the subs' inputs will present a completely negligible load to the amps. HOWEVER, it is still possible that the presence of the subs is causing the problem (see below).
Also, I believe that what Drpat is referring to when he says "effectively reduce it to 4 ohms" is the fact that in bridged mono mode a given speaker impedance will be seen by each of the two output stages of an amplifier as 1/2 of that impedance value. That is because for a given output voltage that is produced by each of the two output stages twice as much current will have to be supplied by each stage as in stereo configuration, since the "other side" of the load impedance (i.e., the speaker terminal that is connected to the other output stage) is being driven to an equal and opposite voltage by that other output stage.
DrPat, can you describe EXACTLY what the EAR instructions are, for using the amp in mono mode? I couldn't find a manual online. And I'm not sure if when these amps are used in mono mode they are being bridged, or if the two channels are simply being paralleled (those being two different things).
And also can you describe EXACTLY how the powered subs are connected to the system? If they have speaker-level inputs that are being driven by the EAR amps, as I mentioned above their high input impedance will present a completely insignificant load to the amps. However, depending on how they are connected, and on their internal grounding scheme, and on the internal grounding scheme of the amp, they could very conceivably be responsible for the problem, if the amp is bridged. That could occur if the minus/black/ground speaker-level input terminal of the sub is connected to an output terminal of the amp which drives a signal when in bridged mode, as opposed to being grounded.