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.

Sure sounds like a problem with the 7500 ground system. Have you checked to make sure the ground plug is functioning correctly. Too bad it is out of warranty although Outlaw will, I am sure, help you troubleshoot it.

Is the hum coming out of the speakers or the 7500 itself and does it come and go in a pattern?

How would I check the Ground Plug functioning properly?

The hum is only coming from the speakers - not the unit. With only the amp turned on and only the speakers connected it is fairly low hum (almost ear pressed to the speaker low) but once the-prepro is plugged in and devices are added via HDMI connections it get's louder and louder. It does not seem tied to the cable input from DirecTV satellite, as disconnecting/reconnecting the cable makes no difference.

If it is the amp I will flog myself for not pursuing more deeply and settling for the 'balanced cables' fix.

Use an ohmeter to verify the continuity between the ground plug and the connections inside the amp. You can also just check against the cabinet of the 7500 on an unpainted portion. If the continuity is variable or the meter reads more than say an ohm you have a problem. The actual reason a ground loop exists is a difference in the various grounds for your equipment. If they are different a voltage is set up and you get noise in the circuits.