There are many threads on Audiogon where you can get a feeling for the sound of a CAT SL-1 preamp. I have a CAT Ultimate Mk I and have commented on many threads. The sound of a CAT is full, sweet, but not overripe midrange, very dynamic, great bass, perhaps better than any other preamp that I have heard which is, at the same time, deep and powerful. Upper mids and highs are never sharp nor bright. Highs are detailed, clean, but not overly forward. Overall, the unit leans very slightly to the warm side, but has excellent overall balance. It is a unit which is very difficult to make sound bad. It does benefit from careful NOS tube rolling a lot. To summarize: smooth, sweet, lush, but not overripe, delivers the entire frequency spectrum, very dynamic, particularly in the low end.
Faults or flaws? There are preamps out there now which are more transparent, image better and which are nearly noise free. The CAT is not a laggard in any of these categories, but it is not at the top of the list on any of these qualities.
Overall: IMO, still one of the best units out there. It exceeds units that are often talked about on Audiogon. It seems that it suffers because it has been around, in one form or another for so long, so people don't really consider it. The most negative thing that I can say about it is that there are other units which do individual things better than it, but, at least with the Ultimate,
it should be evaluated in view of its overall presentation. There are few units which clearly beat it on an overall basis.