You should try out silver plated copper for he HD-650. I bet it would be a perfect match.
With a little work you can find some Belden 1804A that's in a Starquad configuration and is silver plated copper. It's fairly cheap too and it already has a covering, so no sleeving is required, just some heat shrink on the Y-split.
I think for the HD-650 it's best to find a very neutral cable that doesn't add any extra warmth. Mogami and Canare does. Don't know why or how.
What's funny is that silver plated copper from ALO is actually less bright and more neutral than all the copper wires I've tried, including stock. Only used it with my Q701 though.
Another wire that would be perfect for the HD-650 is Belden 1192A. It's not even remotely warm and kind of bright. No rolled off treble like with the Canare.