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I have an issue with RFI interference with USB audio on my PC, which I can only assume is down to the PC case as it’s remained present through multiple powersupply, motherboard and audio interface changes! Fortunately FireWire solves the interface problem as it’s also electrically isolated (and old school midi, which is optically isolated).
Dramatic_Hammer-
Very common with USB audio, it's the quality of the interface since the USB standard doesn't really call for the level of isolation the USB interface needs, hard to blame the PC since it's an RF nightmare inside the case.
I've got quite a few pro level interfaces but one of the nicest 'budget' ones was a Gustard. Simple USB to AES convertor but very good isolation. The Gustard DACs are a lot more money though.
Increasingly speakers use AES inputs anyway so I've found that USB->AES is a useful type of interface.
Airhead
Mac optical out has some speed limits built into it. Optical connection quality can be affected by the cable too, not least a poor quality cable can just fail on and off. The big advantage of optical is electrical isolation. These days it's less usual to have the kind of problems that require the computer and dac to be isolated.