I did the whole "unplug and start again" thing with both audio and power connections, and
found that no hum was present on the audio signal unless any of those three particular
units was plugged into the power. No other units in the rack hum. Bizarrely, when the
offending units are plugged in (again, power only, no audio connections) there's hum on
the mixer's Aux send 4 too - which means I get a permanent reverbed mains drone...
Removing the three offending articles form the power supply cures the problem.
All my cables are home-made to fit using Van Damme cable and Switchcraft/Neutrik
jacks. I've run the mains leads for the rack kit up one side of the rack tower, and kept
audio away and not parallel, as much as possible.
--------------------my nerdy synth tech blog