Ground loops are bad, RFI (RF Interference) is even worse. I was the maintenance tech (and staff steel guitar player) for the old "Big K Records" in Kansas City (north) MO. It was in an old building about a block from an AM radio station transmitter site. I worked and worked on it and got most of it out of the recording system (what was left was low enough to not cause any problems) but I could never get it all out of the headset feeds. Lots of shielding and bypass capacitors were used. It was an old Tascam tape and Tascam console studio with an Ampex for the two track masters.