If the mids were a touch more forward/more present in ratio to the bass in the M50s, would you like that better? If so, that seems to me like you might even like a little more neutral phone and should steer away from the bassier headphones like the X1 and M100. One can be an SQ bass lover (good quality bass and deep extension) and not want heavy bass emphasis that overwhelms other frequencies
If you would like to stay with a bass signature closer to the M50s, I would suggest HE-400. They are quite an amazing upgrade. Incredibly rich bass detail and resolution of other frequencies. Much better soundstage. For headphones that are probably somewhat similar to the M50 bass amount with a bit less of that treble that you commented about, B&W P7 would be worth looking into. Apple stores carry them and often have them on display if you want to give them a listen.