Alex Tech

Yeah open back is generally better when you have no noises around you. 
 
I wouldn't get the M50Xs you are way better of with the 598s.
So either 598+Fulla 2 or 6XX with Fulla 2 if you want to spend the money and are fast enough lol. 

afaik the Fulla 2 is quite a bit better so i'd go for the $49 extra. 
 
But come back to my initial question, do you need it to be closed? Because the 6XX will leak a lot of sound and let it all in aswell.