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Interesting, I’ve not used a pro-level USB interface with this PC to be fair. I suspect a grounding issue with the case, as sending midi over usb will cause the issue to show up in the audio output of various bits of kit, even if that’s coming in on balanced analog cables (not using the cheap baked in usb audio interfaces).
Dramatic_Hammer-
It's part of the design of PC power supplies that a small amount of current leaks to earth. It can cause problems with RCD's in large offices.
Weird that the midi is causing it too but I suppose it could be a ground hum due to earth differences between the midi and audio connections.
One of my favourite firewire options was the Motu 828, they are easy to pick up now although they had some issues with the 2 inputs on the front everything else is pretty good and the aes output stands up to some expensive stuff.
Running balanced cables is only sorting out any interference picked up by the cable, if there is noise in the signal when it's sent it will be treated as signal.
Airhead
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.