a similar, if not exactly the same, instance i stumbled upon with my babyface, something i still blush about, shameful.
i'm sensible to the smallest current drift, for some reason: i can't use a macbook pro without the fully-earthed power cable, the short two-prong stick-in always left my hands with this unpleasant fizzy sensation...
i'm obsessed by correct wiring, i test cables before using them, solder my own ones and consider switching to the same connector of same manufacturer, but the version with different contacts' plating...
...and i'm about just as concerned, or paranoid, to ac mains distribution, no plastic multi-socket strips unless i've seen them inside and checked they're solid-wired, if i don't have one of those industrial knürr beasts readily available...
well, what you're sharing here of your experience happened to me, and i was so damned sure of my own kit that i included in my assessment also the part of the system that wasn't under my control: wall outlet.
the whole house ac system is neatly grounded, i know exactly where the earthpole is stuck into the ground, which i water from time to time, if it's a hot and dry summer. so i doubted the equipment, not the house system.
and i was wrong.
just as MC explained.
the wall outlet i had been plugged into, and for a couple of years at least, had a loose screw on the earth contact, and in its actual position it wasn't linked.
as unlikely occurrence as it seemed to me, it happened, and happens.
not i, but the electrician simply plugged its tester-meter into the receptacles, and came out with "but of course, you're not grounded".
it took as much as screwing it out of the wall, re-tightening all screws, and finally the good, compliant, and safe house grounding system was existing at my audio desk, too.
and i felt like a total idiot, for having thought that if it's good anywhere else in the house, it had to be good here, too.
since then: i learnt (the hard way) screws turn loose with time and current, for power lines have their own ways to vibrate (and at 50 Hz here in Europe, i suppose, as well as 60 in US, J, cruise ships and other 110V mains systems).
and i learnt maintenance on power distribution isn't a way to rip you off some money, if done effectively. even if it just means tightening three tiny screws in each wall socket and single switch in each switchbox, it would have avoided me this very experience, and save you if the worse comes to the worst.
and i now have it (professionally) done, and steadily, here, every other year.
thank you for sharing your experience, i never told my own before (because i feel shameful at that).
even if i can only use eighty lines of text in lieu of MC's three or four ones, speaking of (my own) low efficiency: he got it as right as it comes.
added measure of comfort: i wear those famous German-made, cork-sole sandals when working audio or playing, since i found out that they also exist in an anti-ESD version (that's approved, and worn by collegues of mine, at our company's electronics department).
oh, and about forgetting things: my BabyFace Pro runs on its own PSU even when connected to computer.
PSU isn't grounded, of course, but provides a neat failsafe protection in case usb cable is inadvertently pulled off (as this will only stop audio, not pop all speakers out of their baskets). and because the RME original PSU it's so neat and good, with its lockable plug, i gifted it to my BFPro on last year's xmas!
