Matt, if I followed, the LFR DSP appears to be the only factor causing the hum. All your changes, including a different amp and pre-amp, didn't affect the hum problem as long as the LFR DSP was still in the circuit.
There isn't any apparent way that simply inserting an additional component(e.g., the DSP)in the middle of a circuit could create a ground loop which previously wasn't there. Therefore it seems that the problem is likely to be internal to the DSP, which would need repair or replacement.
