Very interesting to here your response, I’ve also gone through to the hd 660s and I enjoyed them a lot but definitely missed the bass of the m50x’s and also found myself getting distracted by the soundstage of the sennheiser’s (even though it’s not known for a big soundstage it’s bigger than m50x).
What’s also on my radar is the AKG closed backs which are pretty good apparently.
For a while though I have been without headphones and rather just been into IEM’s, there’s some really awesome IEM’s going around that perform amazingly for the price.