So this issue begins almost 7 years ago (5/06) when I replaced my 950/750 combo with a 990/7500 combination.

When it first arrived I spent untold hours trying to diagnose a hum in the system, which seemed to be attributable to a ground loop though I was unable to identify the source. Eventually I thought I resolved the issue by switching to balanced cables from RCA cables.

Fast forward to last month when I purchased a new 975 and a 7125. (The intent was to use the 975 with the 7500 and move the 990 to another room with the 7125). Upon switching back to RCA cables with the 975/7500 combination the hum re-emerged. Once again hours have spent trying to isolate the ground loop to no avail. Every component on the circuit was removed and even with nothing but the 7500 and 1 speaker connected a hum was noticeable. Adding the 975 and even a single component increased the hum.

I assumed the only option might be to purchase a pre/pro with balanced options but decided to at least see how the 975 sounded with the 7125. Upon moving the RCA cables and speakers to the 7125 the hum ceased - not even the faintest sound.

So my question is - does this mean my 7500 is actually faulty and always has been, or might there be another explanation for this issue? Should I still be looking for a ground loop?

The serial number of this 7500 is:7500D00093, not sure if that means it was the 93rd unit produced.