I have the 6XX’s( not the 650’s ) and the Edition XS’s and yes 2 completely different headphones. The big problem I have with a comparison is their difference in amplification requirements. I for one believe if you have not heard a 650/6XX from a Bottlehead Crack you have really not heard them. I have several amps but for me only a few that I think are good with the 6XX’s .
First is the Drop Cavalli Tube Hybrid. It is not a matter of power just that it meshes with it more than say a Magni or Atom.
The Next is a iFi Zen Can Signature 6XX just Awesome the iFi team hit it out of the park with this amp. The EQ and XSpace are done so nicely without being overbearing.
The Bottlehead Crack is the best I’ve heard the 650/6XX’s without a doubt. Some of the best mids ever. I do have a affinity for a Tung Sol 5998 as the power tube.