"the best match for a cat amp is a cat pre since they were made together"
I respectfully disagree. The "best match" for one person or one system of components may be very different for another. As an owner of the JL-3 Signature amps, I put a lot of effort to listen to the CAT Ultimate II vs. the Aesthetix Io/Callisto in my own system for several weeks. And this was with a lot of trials with the stock Sovtek tubes as well as various Mullard and Telefunken 12ax7 and 6DJ8/6922 combinations. The pros/cons of each preamp with the JL-3 amps would be up to each person to decide which way to go. The word "best" simply does not apply here. It comes down to finding a balance of tonality, dimensionality, resolution and dynamics with the assembled system of components that closely matches our own set of sonic priorities.
On the issue of the CAT excelling in the mids, this is neither the case for the CAT Ultimate II nor the JL-3 Signature amps. Both of these products excel in dynamic contrasts, the ultimate in resolution across the frequency band and coverage at the frequency extremes like few if any other tube based amps or preamps. However, when it comes to portraying the ultimate in midrange layers and textures, the Aesthetix Callisto/Io combination and Atmasphere amps are a step ahead here.
I would not label the CAT midrange sound as thin but it does lack some of the fullness (not fat tubey sound) and harmonically rich presentation that many other tube products convey so well. And this is very evident through my SoundLab A1 speakers.