I went through something similar with an amp. I tried dozens of fixes, from cheater plugs, extension cords, turning off all the breakers, different outlets, inexpensive filters, expensive filters, ground loop eliminators, DC blockser, power conditioners, power regenerators, isolation transformers, you name it. Nothing worked. Ultimately, sometimes it comes down to the electricity coming into your house and how it interacts with the component. I hope you have an experienced electrician, because most don't measure/don't know how to measure/don't believe are important, the kind of things, to vanishing small levels, that can be causing the problem. And if he does identify it, good luck getting your local electricity company to address it.
I'm also here to tell you that if you buy a similar priced amp from a different company, there's every likelihood that it won't hum.
As much as we like solving a problem once it's been identified, sometimes the expedient thing is the best route (i.e. I hope you haven't exhausted the store's return/refund policy).