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It's relatively common on synths to have to replace them so there's a decent aftermarket supply. Mostly it depends on how good you are at taking things apart and putting them back together without breaking stuff. I've never been inside a Denon but I'm guessing being a half rack size is going to mean it's packed with electronics.
I always suspect capacitors before anything else. Capacitors going bad have been the cause of most of my hifi gremlins in the last 15 years.
Definitely "wear and tear" over dimmer feature. The display until recently used to work fine. Sharp and bright. Now I'm lucky to start off with something "dimmed" then after a few minutes all backlighting is gone and it remains dark. :(
I haven't contacted any specialist service shops about it yet but I'd like it to work (ideally).