I've never heard of anyone who was sorry that they upgraded a MKI to a MKII. If you're referring to the upgrade from MKII to MKIII, I attended an A/B comparison between them in a highly tweaked system. The clear concensus was that the MKIII is more resolving -- there's more detail (which is saying a lot, as the MKII is no slouch), but the highs seem a bit accentuated compared to a MKII, which can make the amp sound a bit bright. On perfect material, it's a better amp, but the MKII is definitely easier on the ears and will do a better job with less-than-perfect recordings (most of my collection). Three of the attendees (including myself) have MKIIs, and I don't think any of us intend to upgrade. Hope this helps.
