I don't have one as of yet, I'm afraid, but I haven't forgotten.
Ultimately there's going to be a lot of interference inside a PC case, and the software solutions are in place to counter that. It is unfortunate that you've not been able to get the cancellation to work properly on the Phoebus, although it's good that you've got it working on the ZXR. I don't think the 980 would be considered defective as a result, there will always be interference coming from a high-power graphics card and the sound card software should be able to filter it out, as you've said.
If you want to eliminate more of the interference then USB -> DAC is probably the best solution, getting all of the electronics out of the way, but obviously it's up to you how you approach it.