I am wondering if anyone out there can tell me the changes made in the three versions of this unit. I am aware of some of them but am looking for further info. And if you just want to tell me that brand "X" pre amp that you own is better anyway, please don't waste your time responding.
I know this comment is not exactly responsive to your question, but I can't resist. In the late 1980's I bought the first version of the ARC SP-9, and it was the first really "serious" preamp I owned. What a MAJOR disappointment it proved to be. After all the glowing reviews of this preamp in the major audio journals, I really expected a significant improvement in my system. WRONG!! From that time forward, I have always read reviews in the hi-end audio publications with a very healthy dose of skepticism (particularly reviews of ARC's product line).

I kept the SP-9 for about 6 months, hoping that some other equipment changes in my system would let it show its stuff, but it never happened. The Mk2 of the SP-9 version may have been better, but by then ARC had lost me as a customer. Your post does not indicate the reason behind your question, but if, by some chance, you are considering the purchase of any of the SP-9's, I would strongly advise you to buy something else. There are many tube preamps, and even many solid state preamps, that run rings around the SP-9.
I own the first version and have tweeked it with Infinicaps.
I am just looking for ways I could improve it(not because it is dosen't sound good in my system).The last post is not the sort of thing I am looking for, but thanks anyway.I happen to like mine.
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.