Hey,
So, some thoughts.
If you have some standard electrical wire - i.e. the kind that runs inside the wall - you can use a piece of the ground wire, cut it down and make a jumper wire for the XLR if using SE input. If you have any music on your phone, use a 3.5mm to RCA adaptor and run it directly into the amp. At least you can isolate where the problem actually lies.