I've owned a Convergent Audio Technology SL-1 Signature preamp for about a year, and aside from problems with residual hiss--a byproduct of its high linestage gain--I love what I hear. The CAT has the unique ability to sound highly transparent and utterly musical at the same time--from my experience, a rare feat. It's also built like no other preamp, 60 pounds of heavy steel with internal damping material and an outboard power supply. I've always felt I've gotten my considerable money's worth with the CAT.
Mr. CAT, Ken Stevens, is another matter: he's priceless, an interesting and opinionated man who is constantly tweaking his wunderkind. This ensures that his preamp will always sound the best it can, but all his work is a problem for those of us who buy a CAT only to find that it's changed a month later. More than any other piece of equipment, the CAT preamp is tied to its serial number. Keep this in mind if you look for a used unit.
Although the CAT preamp is in constant evolution, only recently has its model designation changed, to (big breath) the "SL-1 Signature Mk II." My CAT is one year old; it has the most obvious improvements that have been worked in since the CAT Signature's debut: heavier bottom plate, and a new umbilical from the power supply to the preamp itself. I've recently had the chance to compare a new Mk II with my original unit, and from a purely visual standpoint, the differences are slight. The circuit boards in the power supplies have all the same markings, but the board in the Mk II seems to be made of a different material and is perhaps thicker. In addition, the one bright-blue Phillips capacitor in the power supply has been replaced with an IC cap, the same brand used liberally throughout the preamp. I couldn't spy any obvious differences to the board or components in the preamp itself, but I did notice that the wiring in my older unit is much neater, with less slack and more calculated use of wire ties. This, of course, is due to manufacturing variation--if only sloppy wiring were all it took to make a great piece of equipment.
Sound: very, very close. The CAT Signature Mk II seemed to have marginally more extended bass and was a little quieter, but these differences could be attributable to its new tubes, especially the 12 AU7s in the linestage. These are the most prickly pair in the CAT and can affect the sound in a profound way. Convergent is now using "German RFT" (what Ken Stevens called them) 12AU7s; if you have an older CAT, you should strongly consider popping for a new factory tube set. The last I heard, Convergent was not going to offer the Mk II changes as upgrades to older units, but by buying new tubes, you may be getting the bulk of the Mk II upgrade in the process. Oh, I did forget to mention the most considerable difference between old and new CATs: a $1000 increase in price. Who says that high end equipment isn't an investment?