I also had the K550 some weeks ago (i got perfect seal btw), it (AKG) sounds brighter, less bassy and with recessed (lower) mids. The bass was also slower and less detailed. It has more of an upper mid peak which fit vocals nicely even though some parts of the mids were lacking in quantity. The X1 has more to offer in the lower mids and slightly less peak in the upper mids, more balanced for my taste.
What i really enjoy on the X1 is the soundstage and how the layering/depth works with the amount of bass the X1 has to offer. For example you get a giant drumkit playing behind the vocals but they never get in each other's way. Soundstaging like this is usually only found in speakers. In all other bassy headphones i heard, the bass would overshadow the rest and ruin the soundstage depth making the bass too present.
In your opinion, is the Fidelio X1 an open, semi-open, or closed headphone?