Can't tell you anything about the SL1 Ultimate from personal experience but only what Ken Stevens told me in a phone conversation with him. The major upgrade in the Ultimate over the Sig Mk3 was the marked improvement in timbral accuracy. He accomplished this thru significant changes in the power supply & associated filtering. So he informed me.
I own a SL1 Signature Mk3 & I can tell you that it is very impressive. Impresses me each time I listen to it! One of the major reasons I bought this pre was for its phono stage!! I have heard the Spectral DMC20 in my system. Excellent ss pre but doesn't have the tube magic of the CAT. Otherwise, a very fine pre despite its age. I know that Ken has the highest respect for Jadis. Haven't heard one just yet.
Now, I don't know which Signature model you owned (there was Mk1, Mk2 & finally Mk3). The Mk3 version was the best. I think that when they upgraded from Mk1 to Mk2 they changed the V1, V2, V6, V7 tubes to 6922 from 12AU7s. Ken tells me that this change made a world of diff. for the better + improved tube reliabilty even more. Maybe you had the Signature Mk1 w/ 12AU7 input tubes??
I personally feel that the SL1 Ultimate Mk2 will impress you a lot but only you can decided once you hear it for yourself.
FWIW. IMHO. YMMV.