Weird Ground Loop Hum

I'm having a weird ground loop hum and wondered if anyone had some insight.

Marantz AV7005 AVR
XLR cables (Audioquest Diamondback) to 7 channels of Krell amplification (Krell Chorus 5200 + Krell KAV 250a)
AudioArt SC-5 SE speaker cable to B&W Nautilus 800 L & R mains
RCA cable to SVS sub
HDMI x3 (Audioquest Carbon) from DirectTV DVR, from Roku3, from Samsung BluRay
HDMI x1 (Audioquest Cinnamon) to Samsung LCD TV

I just added the Krell Chorus and dramatically decreased a very annoying ground loop hum noticed ONLY in L/R mains.
It is eliminated (beautifully silent) when signal sent from the DirectTV (surround modes) and from the Roku (Stereo mode only).
A present and mildly annoying hum occurs when the BluRay player is used to play a CD in Stereo mode only (I didn't try playing a DVD, BluRay disc).

The weird aspects:
The hum does not immediately go away when the source is changed away from the BluRay player, even if the HDMI is removed and the power discontinued to the player.
The hum is present when multiple HDMI inputs are tried (I did not try any HDMI that were in use by the Roku or the DirectTV).
A hum occurred when I used the Audyssey microphone, and then went away when it was disconnected.
A hum, while not originally noticed in surround mode, "crept in" after about 30min, after checking various settings from Audyssey calibration.

They are a pain. Try using a cheater plug, the cheap 3 prong to 2 prong adapters you plug into the outlet. It may be a grounding problem.This is common when you have more than one outlet suppling your equipment. If the cheater plug works,thats your problem. You can then ground the outlets to the same ground. Being you are running a AV system, the ground loop problem might be coming from your Direct TV. I had this problem but with cable. I went to the cable splitter junction. On the side their is a small screw for grounding. I ran a ground wire to the nearest outlet and grounded the cable into the home electrical .Boom, my problem was solved. Another culprit might be a bad cable ,or audio cables lying on or to close to power cords. My best hope it was of some help. Brian