I think he's talking about the "one end only" or "telescoping ground" connection scheme. This is where the shield around a balanced cable pair is connected to ground (pin 1) on one end and left open on the other end. This serves to break one ground loop path, and sometimes does the trick. It worked better in the days of analog system, when there was always an audible noise floor, than it does in today's digital systems, which we expect to be dead quiet even at unreasonable system gains. It's worth a try.
A couple of questions, though. Since he mentioned a line level signal, he must be sending something down that snake that's powered from the AC line. The buzz could easily be caused by a differnce between neutral and ground potential at both ends of the cable. A good way to check this out is to run a big orange extension cord upstairs and power whatever active equipment is up there from the same receptacle as the control room. If that fixes the problem, the real solution is going to involve some electrical wiring work.
Are the inputs and outputs truly balanced on both ends? If not, then there's bound to be some hum pickup in the cable - that's the low frequency hum. A good test for this is to disconnect everything on both ends except one cable path. Connect a mic at the upstairs end and connect the downstairs end to a mic preamp. This will be balanced all the way (assuming no wiring errors in the patchbays) and should be quiet. A transformerless condenser mic is probably the best test tool for this since, if there's a strong hum field up there, a dynamic mic element or transformer could be picking up the hum just like a guitar
pickup. That's not a problem with the wiring between upstairs and
downstairs, that's just a problem with the mic in the room. Get
someone to walk around the room with the mic and see if the hum
Problems like this almost never have a single, guaranteed-to-work
solution. They have to be solved methodically, starting with the
simplest connection (a direct mic to preamp through the cable) and start building things up until the problem appears. Then you know where it is and you fix it.
I'm really Mike Rivers
If you need Mikes email address drop me a line. MooseAudio@aol.com