So after meeting a member @234537 at the Cambridge, UK, HeadFi meet, I had to get myself a pair. I'm a convert!
I have a Stax 507 setup but wanted to upgrade my Sen 580s and borrowed a friends 650's they were good, but not 'distinct' enough from the 580s IMHO to make a change. However, the W1000 is an amazing phone for the collection. Thank you HeadFi community for bringing it to my attention. I'm going to do the direct comparison between the 580 / 650 / W1000 and write it up but right now it's just great having this phone in the mix.
Driven right (Chod Hugo) it's a clean and engaging delight.
Congrats! I have the HD650 as well. I think the two headphones complement each other quite well due to their differences.