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.
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.