Exposed tonearm cable source of hum??

I run a 0.65 mv. output van den Hul Frog through a tonearm that features a looping, exposed (in the sense of lacking any real shielding) tonearm cable such as are found on some linear tracking arms, VPI arms, etc. (brand of tonearm to remain nameless so as to keep passions to a minimum). The tonearm cable becomes shielded after the "loop" and runs to a Rowland Cadence phono stage that features a built-in step-up transformer, i.e., I have a lot of gain. I have had a consistent hum with this set up in two different apartments. After consulting with the manufacturers and trying nearly every conceivable fix, I narrowed the problem down to the cartridge / exposed-loop portion of the tonearm cable which is the sole source of the hum (everything beyond the "loop" going in the direction of the speakers is dead quiet). Because the cartridge leads are connected properly, my guess is that it is the five or so inches of looping tonearm cable that is introducing what is a quite audible hum.

Does anyone out there know if such exposed tonearm cable "loops" are indeed known to be sources of hum? If yes, is there a fix (I can't imagine what that would be) beyond switching to a different tonearm cable design (which would mean, practically speaking, going to a different arm)? I have been listening to a lot of chamber music recently and the hum is starting to really bother me.

Thanks in advance.