I'd recommend the d5k too.
The d2k and the k550 would be a different battle. Musical enjoyment? There's quite a lot of instances where the d2k is not enjoyable at all. And switching from the d2k to the k550 would sometimes make me go wow!! I was missing so much of the music, and with the more clear cut midrange of the k550, sometimes the sound would come to life. A great studio doesn't mean a bad music can.
The d5k exhibit this problem far less than the d2k, as they don't really have this veil and overly dark presentation. They're not grating and sibilant in the highs either like the d2k's are.
So yea the d5k is what you'd like better