I have owned all three versions of the SP-9 over a period of nearly 10 and have been very happy with each of them.I cannot agree with the negative comments above. The SP-9 was for many years the best selling preamp in this country (world?). Needless to say there are many thousands of happy owners. JGH and others did not like the mk1 because it didn't have the typical warm tube sound. By todays high end standards even the mk1 is a bit romantic. The mk1 is best with tube amps and the mk11 is warmer sounding but both are fairly neutral, in my opinion. The mk111 is a higher resolution piece with greater detail and tighter bass.
I traded my mk1 for a mk 11 and then had it updated by ARC for $750 which I thought was rather steep. The main differance between the mk11 and the mk111 is the coupling caps. The Wonder caps are swapped for MIT caps. If you have already replaced the caps with Infini you probably have a better sounding preamp than the mk111 for a lot less money! Other changes in the mk111 are aluminum top and bottom covers (careful you don't bend them!) and a new grounding scheme where all of the input and output jacks are isolated from the chasis and mounted to a piece of glass PC board. This is, I'm sure, very labor intensive and adds significantly to the cost of the upgrade.
I traded my mk1 for a mk 11 and then had it updated by ARC for $750 which I thought was rather steep. The main differance between the mk11 and the mk111 is the coupling caps. The Wonder caps are swapped for MIT caps. If you have already replaced the caps with Infini you probably have a better sounding preamp than the mk111 for a lot less money! Other changes in the mk111 are aluminum top and bottom covers (careful you don't bend them!) and a new grounding scheme where all of the input and output jacks are isolated from the chasis and mounted to a piece of glass PC board. This is, I'm sure, very labor intensive and adds significantly to the cost of the upgrade.