60 foot HDMI run with Monoprice RedMere cable picking up noise?

Assuming you are talking about the Redmere HDMI cable, that can't add noise as you describe to your system. Let me explain...

A group loop hum on a speaker would be 60Hz audio. That happens when the AC frequencies bleed into the audio. Other hum frequencies have similar beginnings.

The HDMI signal does not contain any analog audio. It is only digital. So, for the power line to cause an audible hum in the HDMI line, the AC interference would have to find the part of the HDMI bitstream that had the audio information, convert itself to digital, sum the newly created digital information with the bits that were already there, modify the encoding scheme so HDCP would pass and then repeat the process continuously. Obviously, this isn't going to happen and random bit errors do not produce a continuous hum (or even a hum).

However, I can give you a possible explanation. It is possible that by connecting up the HDMI cable, you've re-created a ground loop particularly if the ground is not the same where the Epson is plugged in versus the Marantz. To see if this is true, run a long AC cord from the Epson and plug it into the same AC socket as the Marantz. If the hum disappears then that is a likely explaination. Unfortunately, that would also mean that any HDMI cable would do the same thing.

A second possibility is that your speaker wire is now close to an AC transformer of some type and that is feeding back into the Marantz.

See what happens if you now remove the transformer that you just added. Unfortunately, these ground loops turn into science fair projects very quickly since they are hard to pin-down.
Edited by alk3997 - 1/18/13 at 12:13pm

Yep, sounds like the ground loop is back. Nothing you can do to the cable will help that. You have to break the loop.

If it is a ground loop, one way to break the loop would be to use a fibre optic HDMI cable. But they are pricey.

In the event that the Marantz is now picking up the noise from the Jennson. You'll also notice that the Marantz (mostly likely) doesn't have a ground plug. Look at where the AC cable attaches to the Marantz and (I'll bet) you only see two contacts. Its ground is based on what is attached to it.

The AC noise is getting into the receiver somehow and it isn't easy to figure out where.

I might also try disconnecting the speakers and listening on headphones to see if you still get the hum. Then reconnect each speaker one at a time. In the old days, I'd also say to check your phono inputs but that probably isn't even possible now.

That's great to hear! Glad we could help.