If you have a spare speaker you could substitute it for you center as a test.
In my last house which was older 1960ish I had to do all kinds of dumb stuff because of noise in my bedroom system. I had to run a separate extension cord from the hall way outlet into the bedroom for certain things like all the wall warts and used ground lift so no loopback . My newer house is dead silent , has a really well designed electrical system. Can you run a cord from another room and only power up the pre/pro and the amp with all else turned off. Like you say maybe some EMI.
