There's an ad for a First Sound from member PF7, and he says in his ad, "The sound and detail is on par with CAT tube preamps -- which is the highest praise in my opinion."
I've never heard the First Sound except at shows in unfamiliar systems, but own the CAT. The things that stand out about its performance IMHO are its ability to handle dynamic changes, both micro & macro, and the fact that you get a great phono stage with adjustable loading included that beats most separate phono stages under $4k.
From what I've read about the FS, and what I know about the CAT, both are excellent from an owner support standpoint, with both owners happy to offer advice on tubes, related gear, etc.
As point of disclosure, my CAT is now for sale to help me fund another project. Cheers,
Spencer