That's a good suggestion in general. I have previously tried this and it didn't eliminate the ground loop. I just tried it again now, and it made the buzzing louder.Have you tried using RCA to TRS unbalanced cables? The 308 MKII manual states that TRS should be used for connection to consumer kit...
9. XLR INPUT – Connect professional equipment to this connector using a balanced XLR plug.
10. 6MM (¼") INPUT – Connect professional equipment to this connector using a 6mm (¼") balanced plug. Connect consumer equipment to this connector using a 6mm (¼") unbalanced plug.
This would be a relatively cheap thing to try vs. the isolator you link to.
However, I used an RCA cable with an RCA to TS adapter. But I expect this will be the same result (ground loop) with an RCA to TS cable. If i'm wrong in thinking this, let me know.
I think I've decided to buy the Radial StageBug SB-6 since it does everything I want, is simple, relatively inexpensive, and I think measures well enough for my usage:
https://www.radialeng.com/product/stagebug-sb6/specifications (click graphs for AP measurements)
The alternative which may still have a ground loop problem is to buy a balanced DAC with all the features I want, then sell my D50s. But given the uncertainty and potential cost, i'd rather get an isolator.
Edit: SB-6 Isolator purchased. I'll reply with an update in 1-3 weeks when everything arrives.