I can't tell you how each iteration changed the overall character 'cuz I have not heard many of them. However, I can say that the each iteration made the CAT SL1 pre sound even better & less noisy (from several conversations with Ken Stevens). I think that it would be correct to say that the overall character did not change; rather, retained the CAT "house sound" (if there is one! Again, several conversations with Ken Stevens).
The CAT SL1 was 1st offered in 1985. The name SL1 has always remained since that time with a post-fix letting us all know which iteration of the preamp was available for sale. At any given time only 1 (one) preamp was available for sale by CAT. However, ALL prev models that were in the market were serviced & maintained by CAT. The CAT SL1 Signature Mk I, Mk II & Mk III were offered in the early, mid & late 1990s. I know that the my SL1 Signature Mk1 was upgraded to Mk III by the factory sometime in 1995. The Mk III version was offered until 1999 at which time the Ultimate Mk 1 was introduced. In 2001 (I think) the Ultimate Mk 2 was introduced. The Ultimate Mk 2 is the latest & only version of the CAT SL1 preamp for sale now. Ken unveiled a $15,000 preamp called the "Legend". Production is to start on this this month, if I'm not mistaken.
Prior to the SL1 Signature Mk 1, the SL1 Reference was the model offered for sale. Before the Reference, I believe that it was called simply CAT SL1.
FWIW.
The CAT SL1 was 1st offered in 1985. The name SL1 has always remained since that time with a post-fix letting us all know which iteration of the preamp was available for sale. At any given time only 1 (one) preamp was available for sale by CAT. However, ALL prev models that were in the market were serviced & maintained by CAT. The CAT SL1 Signature Mk I, Mk II & Mk III were offered in the early, mid & late 1990s. I know that the my SL1 Signature Mk1 was upgraded to Mk III by the factory sometime in 1995. The Mk III version was offered until 1999 at which time the Ultimate Mk 1 was introduced. In 2001 (I think) the Ultimate Mk 2 was introduced. The Ultimate Mk 2 is the latest & only version of the CAT SL1 preamp for sale now. Ken unveiled a $15,000 preamp called the "Legend". Production is to start on this this month, if I'm not mistaken.
Prior to the SL1 Signature Mk 1, the SL1 Reference was the model offered for sale. Before the Reference, I believe that it was called simply CAT SL1.
FWIW.