The single ended connection between the Ref6 and H20 appear to be much more cooperative at 300 ohms / 50k ohms.
Wow, that's a world of difference. I believe Ref 6's circuitry is optimized to normally be best from XLR outputs (whether using SE or balanced sources), but yours might just be the rare scenario where SE wins. You might ask ARC what they think of using a Jensen ISO-MAX XLR-to-RCA transformer box to potentially get the best of both worlds here. That still won't address the tube chime issue lol.