I'm having some problems with a contant hum through my rig and an engineer friend of mine said it was probably a ground loop. It's not the guitar(s) or anything else in the room.

I'm running three US rack units and two UK ones so there are two step down power convertors in the mix (the US gear uses high wattage so the cables are grounded ((has the third cylindrical pin)) - I'm in Ireland so we have the same power supply as the UK. I'm pretty sure all of the UK cables are also grounded.

I've been told that I should only have one unit grounded in the rig... is this true? If so, is it safe for me to use ungrounded cables for particular units when the manufacturers specify grounding for safety?? If so, which unit should be grounded???

Here's the setup...
Triaxis (US)
Eventide Harmonizer (UK)
Noise reducer (UK)
Mesa Simul-Class poweramp (US)
Delta 1010 soundcard rack (US)

I know nothing of electronics or power supply jargon so if you know how to solve the problem, pretend you are talking to a child!
Cheers,