The 6xx is the same as the current production 650 - but for colour and cable. However, the current production 650 (silver screen) is not the same as older 650 units (black screen).
When Sennheiser transitioned production from Ireland to Romania some reported differences in the versions. But that was a brief period and current units perform near exactly as the Irish builds.
As mentioned pad wear shifts the tonal balance. To the better or worse is a personal taste best not left to the internet.
Frans at DIYaudioheaven posted a review and measurements on the family tree
HERE.