I have to say the K2 HD is as I thought noticeably better. There is a volume difference the K2 is slightly louder than the 50th which on first serious analysis throws you when you adjust the volume for listening to the 50th but you can actually hear it in the first few bars when the bass run on So What appears the k2 version defines the notes much better...I hear detail and space around that I have never heard before.
If you really crank the 50th version you can almost hear that detail but when the rest of the ensemble come in it is really loud plus the cymbals sound more brittle and harsh.
The K2 version has a fantastic balance of detail, dynamics and smoothness that is a bit unusual in digital some SACD can do smooth but not dynamics as well.
It is a very well mastered dic but I would be interested to hear other opinions on this.