The 6-eye Columbias are, generally speaking, the best you are going to get. I suspect that it is a matter of the master tape degrading over time (which is why Sony and Philips developed DSD to archive recordings in the first place).
The "Kind of Blue" on 6-eye is pressed at a slightly wrong speed. I like the mono mix of this recording myself. I've only compared my "Blues in Orbit" 6-eye to the SACD release, not to any other pressing; the LP is a winner. Another good example of the superiority of the 6-eye release is Brubeck's "Take Five."
The "Kind of Blue" on 6-eye is pressed at a slightly wrong speed. I like the mono mix of this recording myself. I've only compared my "Blues in Orbit" 6-eye to the SACD release, not to any other pressing; the LP is a winner. Another good example of the superiority of the 6-eye release is Brubeck's "Take Five."