So I finally bought the HD 600 and added it to my collection today. I was in conversation with someone on eBay about buying a 2016 marble top, but decided to order new in the end.
On some tracks I prefer the warmer, darker 650, but on others the 600 airiness wins. There's been a few times that added sparkle of the 600 is noticeable, but it has the better vocals and guitars. And that neutrality is magical. On the Gremlin the 600 sounds beautiful just like its siblings. My curiosity would love to hear a JAR650, add some of those custom Malvix Grilles I can't get in the UK, or pick up a 580. However I think I need to stop now with the HD 6x0 no matter how much I've adored exploring them. Although it is going to be fun listening to the 600 more over the coming weeks and months.
As an aside I noticed those Linsoul Tripowin Granvia cables for the 6x0 series are no longer available on Amazon which is a real shame as they were fantastic value for money compared to the custom cable options. EDIT: 22/08/25 - scratch that, suddenly pops up on Amazon again this morning; in a limited time deal for £36. So who knows with that.