I own the X1 and can vouch its fantastic. Demo'd the AKG K550 in a best buy today (was looking for momentums) And unless theres something im just not getting about them I felt they were total garbage, like zero bass (as in literally none present). Now this may be that they were not being amped in which case they may sound like total crap and be a different story when amped. im not familiar enough with the k550 to know how much Amping affects them. The cup seal on the head was not very good ( i think this is a known issue).
I'm in a toss up right now between the HD598 and momentum right now and also vmoda m-100. Leaning heavily to the 598 for the good mids and super comfort. Momentum I'm sure sound fantastic but I think my plan will be to go HD 598 to suppliment my good bass cans the x1's for now and then m-100 later for mobile usage and "just for fun"
Don't know if that contributes to the convo going on at all but thats where im at right now
EDIT: posted my above response before I got completely done with the thread lol, guess I missed the party by a few days.
Thanks for the opinion man. I'm pretty set on getting the momentums but I'm still possibly considering getting the hd598s since they are cheaper and I could use the money towards a better guitar amp(which I am also in the market for).
i pulled the trigger on some hd 598 and are inbound. hoping to get a listen this weekend or early next week. if you can wait that long i can post back here and tell you my opinion. i found v modas in mall this weekend. ill write about them for you here once im off work