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.
If you call Leonard at ARC (phone no. is on their website at he will tell you each and every change, and probably recommend you get the current model LS-25!
I can't comment on the III, but I have owned the I and the II. I don't remember what the actual details are as far as circuit or parts differences, but I remember very well what the differences were between the two as far as their sound in my system at the time. That system consisted of VPI/ETII/VDH MCI, Berning 2100 amp, modified MGIIIA's, all connected with the top Monster cables.

Buying the original SP9 shortly after it was introduced and having gotten a tremendous amount of attention in the audio press was one of the most frustrating yet educational experiences I have ever had as an audiophile. I bought the unit because many of the writers(TAS) that I respected were hailing it as the breathrough "serious" preamp that was reasonably priced and I heard it sound incredible at a dealer in a system consisting of SOTA/Koetsu Rosewood, VTL monos, and Snell A's. Inexperienced audiophile that I was at the time, I did not see the trouble headed my way. The dealer's system was very lush, full and rich sounding; my amps and speakers were lean and tonally very to the yang side of the equation. The dealer should have seen the problem also, but that's another story. Great pre-amp just not in my system.

When I replaced my Counterpoint 5.1 with the SP9, I was shocked. Yes, I gained the image stability and detailing that I sorely missed and the dynamics were quite natural, but tonally the sound coming out of my system was bizarre. Bleached and gray sounding and way too lean. Not harsh at all but totally devoid of tonal colors and lean, lean , lean. System matching was all wrong.

A couple of years later, on a whim, I bought a Mk II. Big improvement I thought, over the original, but I thought the designers had gone a little overboard as far as changing the tonal issues. The sound was now much darker and fuller almost chocolate brown with great imaging but everything sounded closed in with very little sense of air around the instruments and the music did not seem to move as well as with the original. If I had to make the choice today I would probably choose the original and pay more attention to the rest of the system.

I hope this helps in some way and good luck.

I have had an SP9 mkIII in my system now for about 6 years.
All my componets except(of course) the turntable is from
Audio Research. I'm using Audio Magic Sorceror interconnects
and Kimber speaker cable. Speakers are the Dunlavy SCV(s).
I have done a fair amount of tweeking with the preamp and have found that the unit is capable of really very excellent performance under best conditions. Some the tweeks include using isolation devices under the unit resting on a 2" thick granite base resting on sorbothane type pods. I have also found the unit sounds slightly more detailed with the top grill off. Using the Densen Demag CD every 3-4 days also makes a difference.

I've come to the conclusion that system synergy and attention to detail probably has allot more to do with satisfying results than adding the new 'product of the month'.
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.