Hum sound ..is it me or is it you? LOUD hum in system

I sold a classe SSP-30 preamp 2nd week of April. worked fine. I played with it . zero noise at all.
sold it to get a simpler dedicated 2 channel preamp.

nearly two weeks after it was delivered I get an email saying that when the classe 30 is installed there is loud hum...but when its not there is no issue. 

that is all I am told..and "I want to send it back"

im kind of agitated that buyer took his sweet time to get around to checking it out or whatever.  Not sure what to do or what I can do. I told him to send it back so I can see it.

my question is, I read lots of info on grounding loops and funny noises .   because buyer claims he has noise does it leave me any other choice but to take it back?

I used the pre before I sold it for this exact type of reason.  i just didnt take video.   im not to pleased with the extra long delay neither. seems like if you spend that much money on something you check it out right away. it doesnt take a lot to hook up a preamp.

i find it hard to believe a preamp of this caliber would even have ground issues.

I dont know what to do. I feel like I am at this guys mercy no matter what?