M9XX pairs VERY well with all high impedance Senns. Ive got an HD580 and HD800, and it sounds incredible with each. FYI - HD650 is the improved HD600, which itself was the higher end version of the HD580.
I had HD650 and many more. I recommend a more powerful amp if you have the resources. I can highly recommend Bottlehead Crack (with Speedball if you can find one). HD650 loves tubes. As for solid state Schiit Jotunheim.
Only comment on the Crack, it's a high output impedance OTL. Amazing with the HD650, great with the HD800, but you'll have a bad time using headphones with an impedance under 300ohms. In that way, it's a bit of a one trick pony. If you're looking for something more versatile, checkout the Jotenheim, or m9XX.
Appreciate the compliment, no plans for other colorways at this time but we make decisions like this based on community feedback and requests. Thanks for the suggestion!