Spencer, when you said you used ground lifters I assumed you used them on both Amps. That would have eliminated the chance of a ground loop imo.
You mentioned in your original post you inserted shorting plugs in the inputs of the two Amps. Do you get the problem hum without them in the inputs of the amps? (Just the Amps connected to the speakers, inputs open not shorted.)
Another question,
If just one power amp is connected to the preamp does the amp have the problem hum?
You mentioned in your original post you inserted shorting plugs in the inputs of the two Amps. Do you get the problem hum without them in the inputs of the amps? (Just the Amps connected to the speakers, inputs open not shorted.)
Another question,
If just one power amp is connected to the preamp does the amp have the problem hum?