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Never had an issue with this, in fact one of the bonuses is you don’t need to run out long AC lead to FOH with the Multi, Used 2 CAT 5 cables for about a year before going optical, obviously it is pretty hard to get earth hum loop down a bit of Fibre[:D]
The other thing is if you keep to balanced I/O you limit your problems in this area also, I know some laptops used for playback tend to hum as other threads have suggested but generally the use of a Good Quality DI box (some specialist ones for laptops and iPods exist) one with decent transformers in helps loads.
Hope this helps.