If you disconnect both RCAs from your source and the volume control is all the way down, is the hum still there?
Again with the source disconnected and you turn up the volume knob, does the hum get louder?
If your AC plug has a ground and you use a cheater plug to not connect the ground pin, is the hum still there?
I've never used a cheater plug before, but I know my source doesn't have a hum. The hum is only there when I bring the volume up on the 323s amp pot whether or not the source is connected.