question for Luxman L-590AX owners

Lot of this has been covered. This exact integrated amp has been dead quiet since I've owned it, which has been 3-4 years, up until a couple of months ago, when the hum started. It's been plugged into the same outlet since day one. I just now checked the voltage at the wall, 121.2-121.6VAC. Tested several other outlets in the house, all in the 120-121V range. The panel is 10 years old, professionally installed. The PUD service drop is new, although the line feeding that is pre-existing. Same with my shop, with the exception that I wired it. Same service drop to parallel feeds into a 400A meter base, one set of feeds serve the house, the other set serve my shop. The amp has been powered up in both buildings with no difference, so unless the power company has a new issue, I'm ruling out our power.
I'm leaning heavily towards an issue with the transformer in the amp itself, or so it seems to me.
I'd feel more comfortable taking it apart if I could find a service manual.

One problem for me, like any other internet forum, is evaluating the answers that are offered. If it was my area of expertise, that would be great, but it's not. Are there differences in the design requirements for transformers between Class A, AB, etc. Is the heat of the Class A an issue? Are there other differences? I'm talking amps with similar output, of course.
Thanks

The Luxman power transformer core is constructed by stacking layers of thin laminated steel sheets. The transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization. Transformer noise is caused by magnetostriction: means that if a piece of magnetic sheet steel is magnetized it will extend itself, when the magnetization is taken away, it goes back to its original condition.

Each laminated steel sheet with an induced magnetic field. That magnetic field acts between the adjacent plates stretching and squeezing the adhesive and insulation between them. Over time that adhesive starts to break apart and the laminated layers separate from each other slightly. That is the humming noise that you heard. It’s always present, but once the adhesive starts to break it gets louder. These micro-fractures in the adhesive may not be visible to the naked eye.

Maybe this is the cure:

https://www.youtube.com/watch?v=WW-QRar2RQ8

The more current draw through a transformer (class A amplifier) the larger the induced magnetic fields, and thus the louder the transformer hums.

@imhififan This is fascinating.  So is a toroidal core like a spool of one continuous sheet wrapped around itself, or is it built up of an accretion of concentric circular sheets, each one slightly larger in diameter than the previous?

@twoleftears 
The power transformer in the Luxman is a EI type and not a toroidal transformer.

https://6moons.com/audioreviews/luxman4/6.jpg