Hegel hum?

My solution to the nest of wires (I have the Hegel H20 amp) is to seperate all interconnects and speaker wires from AC cables. I put that black foam pipe insulator with the slit in the middle  (cut into several pieces) to seperate all the low power interconnects/speaker wires from AC cables, just slip it over the interconnect and AC cable, cheap and effective.

Sounds like either a combination of cables picking up AC and/or DC gettting into your power lines, several products out there to clear up DC on power lines which make transformers hum as many have noted.

Your system is deserving of power conditioning, I like the Puritan PSM136 conditioner with groundmaster and some Verifi audio plug in thingies, astounding how much improvement that comes from cleaning up the power.

all transformers humm, some very much so. Usually they are bolted in tightly, or have some elastomer in the bolt to quiet the humm. 

You may have an issue where the transformer has loosed up, and is being allowed to vibrate. If it has elastomer it might have been squished out, deformed, or come apart. 

Pull the amp out, disconnect from everything, pull the case, and see if you can move the transformer around. Also check to see if anything holding it down is loose. If it has elastomer, it should be "tight" but not so much so that the elastomer disformed too much, or gets pushed out. 

It's like a 10min fix, also normal.

I recently bought a Hegel H400 and have also experienced a slight hum, normally not audible from more than a foot or so away, but very occasionally it's just loud enough to hear a very low-frequency hum from my listening position. A DC blocker hasn't helped. Apparently, this is completely normal for a Hegel when the power supply is sub-optimal. Their own website has the following info: 

If you are hearing a mechanical hum from the transformer and this also varies in loudness, it is nothing wrong with your amplifier.
Hegel amplifiers have very large unwrapped toroidal transformers. Unwrapped transformers are more prone to picking up electrical interference. Still, we have chosen not to isolate/wrap our transformers, as this would seriously degrade the sound quality of our amplifiers. The humming noise does not harm the amplifier in any way, but it can be annoying.

What you are hearing
The noise you hear is the vibrations in the windings of the toroidal transformers. Imperfections in the copper wires allow the windings to vibrate. We, of course, use the best quality copper and production methods in our transformers, but on a microscopic level, the wires are still not 100% round. That means that even if the windings are completely tight, they can vibrate, and make this humming sound.

Where does the noise come from?
It's the transformer that picks up other sources of noise on your home's electrical system. Typical culprits are products such as induction stoves, fridges, adjustable lighting, heaters, etc. These create an uneven load and thus generating DC back into the electrical system, something the transformer in the Hegel can pick up.

How to solve it
If you hear this hum try to troubleshoot by removing all other electronic components from the mains circuit the amplifier is connected to, to see if the sound disappears. If you can’t find the culprit you can use a DC filter/blocker between the amplifier and the mains outlet. This will remedy the problem.

dave543210 Very interesting info. Thanks for sharing.