WA could have chosen to put too few turns on the secondary of their mains transformer in an effort to not overheat their DC voltage regulators. Under low-line conditions (rural setting, long AC mains feeder, high-draw neighboring farms, etc.), the post-rectifier regulators may not have enough voltage overhead to suppress the 100 Hz ripple. There are photos of the insides of this unit on the WA site. It has a standard IEC fused inlet with no EMC filter, a toroidal mains transformer, and a dual FWB DC supply with what look like standard linear regulators on minimal heatsinks and no air flow. Very standard but thermally suspect. OP's first post reports that the unit hums enough to be audible in phones even with the unit totally isolated and its ground lifted. That rules out any earthing issues, IMO. And it has tested okay at two repair shops which rules out failed PSU caps, etc. OP also reported that the hum varies, which would be the case if the line voltage was fluctuating from, say, a neighboring farm's varying AC load. This is why we want OP to check the mains voltage. If this low line theory proves true, a mains boost transformer of some sort would be a viable fix.
