I have an Outlaw Audio 7500 Amp, and the Marantz AV 7701 prepro.
I'm using XLR cables to connect the output of the prepro to a xover, with five channels and five bass/subwoofer.
With no input, or between tracks and at times of quiet, I hear a distinct 60Hz hum coming from the 5 main speakers, none from bass.
The five are powered via the Outlaw.
I read another similar complaint on AVS forum.
I noticed the power cord on the Marantz has no ground.
All my kit gets power from a Furman AR 1220.
Any others with same problem? Solution? Root cause?
TIA - BSD

60Hz sounds like a ground loop problem coming from the Marantz. You might try grounding the Marantz's chassis to the 7500. On the left side of the 7500, the back screw is marked for doing exactly this. If you have any other questions, feel free to call Ben in Customer Service at 1-866-Outlaws

The more components you put in the signal path the more likely you are to get a ground loop. That is because each has to see the same ground plane to prevent creating a situation where one or more components bleed their voltage to ground at a slightly different rate which results in a voltage potential between them. All A/C components bleed their voltage to ground in the main box where the breakers are located. Both the ground plug and the wide prong on a two prong component end up at the same place in the breaker box effectively creating two ground planes if not electrically the same coming out of the receptacle. Grounding the two components together will eliminate the noise issue in most cases but the are other ground planes being seen by your system too. The cable most of us have coming in from either a networrk or the cable company/Directv is also grounded somewhere which creates a third ground plane which not all of your components see. Usually the only components that see it is the cable box and the AVR/AVP. As an experiment disconnect the cable box from the Marantz and see if the problem goes away. There are cheap isolators that can fix the issue if that is the cause.

I don't have a cable box.
The inputs to the Marantz are CAT 6 network cable, HDMI from HTPC,
The outputs from the Marantz are HDMI to JVC RS-50 Projector, Front L,C,R XLR to active xover, Rear L,R RCA to powered Sub

Everything is plugged into the Furman AR1220, which has a home run 20A pulled to breaker box.

I have attached a ground strap from the Marantz to the Outlaw, no change.

With no signal (no cables) into the Outlaw there is no hum.

I have tried to isolate this by eliminating components, but to no avail

Have you tried connecting the Marantz to the Amp without the crossover? It could also be a bad cable. XLR cables are pretty tough but they can have issues. It could also be a bad connector socket. Try connecting one signal cable at a time (power off) and see if one produces the noise. You could also try the RCA input for signal connect. Is your sub powered from the Furman too? Can you ground the Marantz to the Furman with a piece of speaker wire just to check if the noise goes away (the Furman might have a lug for just that purpose). If you have already tried all this I would steer toward the crossover being the issue and then the Marantz and then the Furman since the amp has no noise with no input.

I've seen this hum occur from ethernet cables. Try disconnecting just the cat5(6) cable. If it goes away then you can move onto some solutions. I personally plugged my HTPC into the same power strip as my amp. Reduced hum by 90%. Then I cheated and defeated the ground on the computer power cord which resolved the hum/ground loop.