Allnic H-3000 hum when mute button engaged.

Calvin, Do you live on the East Coast, USA?  If so, I can strongly recommend Bill Thalmann in Springfield, VA. Bill is a fine person and smart repair guy who is completely honest.

Forgot to note that your Shindo stuff is strictly single-ended.  So I don't know whether you can really do true balanced mode, to test the function of the switch.  (I am not even sure how the Allnic achieves balanced output.  It might be via a transformer, since I would bet the circuitry is actually SE.)

@lewm- you may already know this, but Bill Thalmann is authorized to work on the Allnic gear. And I agree, he is first rate. He restored a bunch of vintage gear for me that give me great joy in my second system. Nice man to boot. 

I am on the west coast, Southern California. 

@dangcaonguyen - I agree that you should speak to seller first before you open up the unit or commit to spending money. May still be worth talking to Albert about your options. Perhaps it is a simple fix, but presumably that’s going to be out of the seller’s pocket. (and you’ll need to find a decent shop near you-- I hate shipping gear if it can be avoided; I actually dropped off my stuff at Thalmann’s en route from NY to Texas).
Assuming the unit is already out of factory warranty period, the main concern- once you reach agreement on approach with your seller, is to avoid a situation where the manufacturer says "oh, you let someone unauthorized work on it, so in the future, you may have problems getting service." Frankly, I don’t think Albert would jam you- and the support I got through David Beetles (and subsequently Albert) has been first rate- they have the relationship with Mr. Park, and it makes it way easier for them (now Albert) to guide you. Hopefully, you get it sorted fast, cheaply and easily. Let us know what happens.
I would think there would be some good techs in your area.