If you disconnect source(s) to the amp, you can isolate the problem. If the amp is causing the buzz (perhaps a transformer issue?) you will hear the sound when all inputs are removed. In many cases just the input (RCA or balanced cables) from your preamp. If you don’t hear the sound it could be further up your chain, like your preamp or source.
Interestingly I had a hum/buzz issue with TWO components I had once, I will not name the company, but both an amp and a preamp had bad transformers. The brand denied there was any problem. When I tried swapping another brand amp and preamp, the buzz went away???
