Apparently the only difference between the HD6XX and the HD650 is the colour and the length of the cable, other than that they are identical.

I read that here: https://headfonics.com/2017/02/the-hd6xx-from-massdrop/

I have the HD598 model here and I love them.

I've never tried the HD650s, are they significantly better than the HD598s?

Man, $200 is a great price!