It's buzzing/humming and I don't know why. I checked the manual for guidance but none of the suggestions helped. I've unplugged everything from it and powered on but the hum persists. I've also tried it away from the rest of my electro stuff but to no avail. The hum isn't really loud but it's noticeable if the music is turned low or during quiet passages. I got it used but it didn't hum initially. Has anybody else had a similar problem? It's equipped with the optional MM phone stage. Could that have something to do with it? I thought about opening it up but I don't know what I would be looking for. Any thoughts would be appreciated.