90493m,
Good to read yet another person enjoying the CAT SL1 Signature preamp.
Like Spencer (who posted before me) I too own the SL1 Signature Mk 3 version. Simply love it! I have never heard the Mk 2 version myself but my conversation w/ Ken Stevens also revealed the same info that Spencer has already posted. So, time & money permitting, I highly encourage you to upgrade the Mk2 to the Mk3. The upgrade should blow your mind!
The Ultimate is supposed to be another step up: from my conversation w/ Ken, the focus in the Ultimate is the power supply (my, what a surprise!) & getting the timral accuracy more right/more true.
Enjoy this preamp - it is a special piece! Yes, there are many people looking for some "colour" in their music thus, do not take well to the CAT.