I did the upgrade and I recommend. I did it last year so I can't give you a detailed comparison but there was a significant improvement. Wider soundstage and more separation of instruments. It is not an expensive upgrade and it will take your performance up a notch or two.
Over at audioasylum, Charles Hansen made it clear that this "MP" upgrade (not to be confused with the filter MP upgrades for the digital players) is *only* for the K-5. To my knowledge, the "e" upgrade is still current production for the AX-7.
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