Makes sense to me. Whatever the issue is with how the device is grounded would likely follow through an RCA connection. XLR will be better at rejecting this noise. If you must use USB I will suggest again the Topping HS01.
I don't know why the HO100 behaves this way. I have tried it connected to different power outlets but the source of the noise must be the USB ground.
I have considered removing the ground pin on the cable but I'm not sure if that would be dangerous or what the path to ground would be at that point.
Yeah the Power ground doesn't seem to have any impact on it, it's coming from the computer's USB connector through the DAC and along the RCA cables into the HO100 and out of the headphone jacks, now... One of my XLR cables i ordered arrived (other one apparently delayed til Saturday) So can only test one channel at a time currently, but with XLR it's cleared up completely on headphones, I've not tried IEMs yet but I'm thinking it'll be the same story there.
Have also ordered an optical USB cable to see how well that works with DACs, curiosity has gotten the better of me there, should also be here in a few days.
Still doesn't feel right spending £80/€94/$95 on a USB cable
but if it at the very least makes the RCA jacks more useful on the HO100 then I'll be ok with that.
Next up is to try and lift the ground connection at one end of a set of RCA cables.
So yeah a set of £5-10 XLR cables are a good option if you're feeding it balanced, I'm using some STAGG branded cables, relatively cheaply available on amazon, have used them before the cable is nice and flexible and the connectors unscrew and disassemble easily.
TLDR if you're planning to use it with the XLRs you're good, if you're wanting to use the RCAs YMMV/Some Jankery may be needed with the cables/Grounding depending on your setup.
Edit: lifting the grounds at 1 end of the RCA cables didn't help(actually made things worse by introducing some different noise so yeah don't do that). Something I should have checked even with the RCAs disconnected there's still a ground connection between the devices RCA grounds, so that did absolutely nothing good, removed the ground(earth) from the power plug and that's removed the PC noise(probably the first thing i should have done), so yeah ground loop caused by the USB and the internal PSU on the HO100, So when the optical USB cable arrives, and i reattach the ground/earth on the power cable that hopefully should be all ok. Hopefully my stupids can avoid some annoyance for others.
A revised HO100 with a ground lift switch might be nice as some other devices have.