I'm not saying it isn't the preamp, it could be, but ground loops are tricky. For instance, I originally thought my preamp was the problem. I had a buzz coming through the speakers, when I unplugged the inputs to my amp, the buzz went away. I unplugged the inputs to the preamp one by one, the buzz stayed. So I thought it must be the preamp. I went to try to float the ground on the preamp, but noticed the buzz got louder when I unplugged the preamp. Strange. So then I thought maybe I needed Cardas Caps, I thought maybe the open RCA/XLR fittings on my preamp were picking up noise. Wrong, Cardas Caps did nothing. Then I unplugged my cable tv main line coming into the house, viola, buzz gone. So I bought a MIT Iso-LinQ to isolate the ground from my cable tv, it worked, no more buzz.
The bottom line is some equipment is more suspect to ground loop noise. There may be something wrong with the preamp, maybe it's just the system. I bought a brand new preamp direct from Cary about 12 years ago. It had a buzz. I sent it back, they could find nothing wrong and sent it back. The buzz was still there. I sent it back and again they told me there was nothing wrong. All I know was when I plugged my Aragon 24K in there was no noise, with the new Cary, there was. I figured out years later, that it was my system, not the Cary that had the problem. The Cary was just more sensitive to ground loop hums than the Aragon. When I had the same problem with an Audio Research preamp a couple years later, I realized that in my system, tubes were more prone to ground loop hums. With work, I could eliminate the hum, but for some reason they always picked up noise, more so than SS preamps. I don't know why.
I don't know if any of this helps, all I can say is try isolating the cable tv line and floating the grounds before you isolate and blame the preamp. It could be a ground loop hum.