Good start, but I'm a little confused now.
1) If you have a PC (Mac, laptop, etc) connected, remove it. Does that help? - YES!
2) Remove all TV feeds (cable, sky, satellite, freeview) from all components, so that you are unable to watch or record TV. Does that help? - YES!
etc...
From 1, it appears that removing the PC removes the hum. But then why does removing TV feeds help - there's already no hum after removing the PC. Moroever, what do you mean by:
6) Power Xbox on - hums like before
...
Step 5,6,7 were repeated for PC and PS3 with same results.
which states that there is a hum, although the PC you removed in step 1 already completely resolved the hum?
Anyway, I'll assume that since you have clearly stated that removing the PC fixes the hum, all we need to do is address the PC. That is, the TV, Xbox and PS3 do not hum if there is no PC connected - the hum is caused solely and exclusively by the PC.
If this is not what you meant, say so NOW, by reanswering all of the questions, starting from a completely connected humming setup, carrying out each step
cumulatively.
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As for the whining - that is something completely different from the hum and is not ground loop related. Rather is is an instrinsic, native property of various audio simulation modes, especially those involved in "widening" sound stages or increasing numbers of channels (e.g. stereo to surround)., The effects can be minimized by tweaking the parameters for the modes causing the problem, or avoided by not using the "add a whine" modes in the first place. In any case, it's a different problem, so let's please ignore it here, by running the amp in standard modes (e.g. without "Theare Dimension" or similar whine-inducers). You can always open another question for unusable mode issues once the hum has been resolved.
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Fixing PC-induced ground loops is well nigh impossible unless you can totally isolate the systems.
If you only need audio from the PC, connect it to the Onkyo using an optical cable, rather than coax or HDMI. This is the cheapest and easiest solution.
If you need video and therefore must use HDMI, try plugging the PC into the same wall socket as the amp, and removing any non-wireless internet connections (we can sort that out later). This can sometimes help - does it?
Ohterwise, the only way to isolate the PC and use HDMI connectivity is to run HDMI over fibre. You'll need to buy one of
these. This is not cheap, and it therefore makes sense to try other approaches first.