what the best phono preamp used with CAT SL1

Well, opinions vary, but some of the best would be the Aesthetix IO Signature, Manley Steelhead, Counterpoint SA-9, to name a few.

Why not just activate the phono stage?

I assume you have the CAT Linestage only, I would look adding the CAT phono section. It is one of the very best available at any price. Other top choices for tubes would be Audio Research, BAT and Herron . Solid state, Klyne Audio Arts, Jeff Rowland and Pass.

Good advice above. However, the investment may also depend upon your TT set-up & how much you want to spend, proportionally. CAT's own phono is very good, and won't require extra cables. IMO, if you want to stay w/ tubes and do NOT wnat to invest in the likes of Aesthetix, insert the phono in your pre. If you prefer to for ss, another two I'll add to the above are, Ayre & Clearaudio reference. There are many others I don't know about, of course...

Maybe it was the rest of my equipment but I replaced the CAT ultimate because of the Phono stage. It may of been it wasn't compatible with the amps I had at the time. It lacked robust and the dynamics I was looking for. I replaced it with a nice priced full function Aloia.

Strange, Snook, I also find that the CAT's line is superior to its phono. As you say, the dynamics... but a good phono all the same.
That's why I wondered which analogue rig is playing here..