Hegel hum?

I've had preamps and amps that were sensitive to DC on the line before.  I've tried many DC blockers and pretty much all of them hurt the dynamics.  The one that I found to work best and not hurt my system dynamics is the Emotiva CMX2 AC Line Filter With DC Offset Eliminator.  It is a great design and does what it needs to do, nothing more.  I have one extra used one with the LED lights not working (a common reported problem) that I could let go for $75 plus shipping if you are interested.  My new Atoll IN200 evo integrated does not hum at all in the same house and location.

My experience with high current Class A or Class AB designs with large power transformers like Krell and Classe is that they normally produce a faint mechanical hum when first turned on as transformer charges and “warms up”.  The hum should not be audible after warmup.  I am not familiar with Hegel in general, but the H400 specs indicate it is a high current design with a large power transformer, so some hum on warm up may be expected.  You state this occurs after warm up. This does not appear to be normal.  I would contact Hegel for recommendations. 

It does not appear you are hearing the hum from your speakers.  If you are not hearing it from your speakers I would not expect it to be  ground loop hum.  If you are hearing it from your speakers, I agree with others to experiment with removing ground loop hum. See the link below that explains ground loop hum and how to attempt to remove it.  If that does not solve the problem, then contact Hegel   

https://www.sct.com.tw/articles/4-effective-approaches-that-eliminate-ground-loop-video-audio-interference

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.