They are connected to different outlets. But I took the sub off of my system and the hum remains. I don't have the third pin for ground here (I don't mean the XLR, but the sheer electrical connection to the outlet), so I use an adaptor - can that be the problem?
And
@Matias , I took all off except for speakers and the hum remains. Even when I turn the amplifier power off, the buzz remains for a while. It's electrical, inside the amp box, and the same noise goes to the speakers.
I only use XLR - I took off the y splitter. I'm waiting for the arrival of my Gustard X16 to use the sub connected there. For the turntable (my other source), I gave up the use of the subwoofer. The turntable (a LP120x with preamp built in) is RCA to Little Bear then XLR to amp.