I have Macintosh ma 6100 and I have been using it for a while combined with a dac through aux input. It always had a low, barely noticeable 60hz hum through both speakers. I just got a turn table and unsurprisingly the hum is more noticeable through phono input. The hum is present without any wires plugged in. I searched these forums and the common advice is to check for ground problems. What would my first troubleshooting steps be in relation to a potential ground problem?