CAT SL-1 iterations

Frank,
FYI, there are two iterations of the Ultimate; the Ultimate, and then the latter Mark II.

FWIW, I own the SL1 MkIII, and have heard the Ultimate mk.II in a different room, but with both using the same speakers, cables, and turntables(Merlin VSM-MX, Cardas Golden Ref, VPI TNT6-HR). One room was the 2005 Merlin room at CES, which had a CAT stereo amp, and the other is my own rig using a Berning amp. Granted, I have had the benefits of likely cleaner power, familiarity, and time to play with the room, but OTOH, the CAT amp is 2x the price of my Berning. That room also had top flight power conditioning that I lack. Not to mention the pros obvious skill at room and equipment setup. Both systems sounded great, and very, very similar. Perhaps more slam in the bass than I got at home, but not a lot of difference. I was asking myself, would an Ultimate be a worthwhile upgrade for me, and the answer was no.
Would be happy to talk offline as well, if you're interested...Cheers,
Spencer

Frankpiet,
In this hobby, there are few absolute truths. I can't prove anything either, but when I was looking to buy a preamp to go with my Berning amp & Merlin speakers, I talked to everyone I could(including Ken Stevens), and asked for recommendations below $4k used or new(including phono stage). No one suggested what you've heard. Ken told me that the improvements going from the MkII to the MkIII were very substantial, and that he strongly advised me to look for a MkIII if in fact I couldn't afford to buy his current model the Ultimate MkII. I concluded from that the improvements from mkIII to the original Ultimate were not that great relative to the incremental costs associated w/the Ultimates.

Many will tell you that Marantz never made a product as good as the Model 9 amps, that Audio Research never made a preamp as good as the old SP6, or that Quad ever made a speaker better than the ESL57. But there are always new models that feature techno improvments. Some ring truer to the music than others, as you well know.
I have tried many other preamps over the years, and have found this to model to be a top performer in its price class, and prefer it to many more expensive pieces. If you are anywhere near Philadelphia, you are welcome to come listen and decide for yourself. Cheers,
Spencer

Andrew,
Yes, the CAT has plenty of gain to drive the .48mv ZYX. Prior to buying the ZYX, I used a Shelter 501II that is .40mv. Both were fine matches. I typically keep volume around 8:00-9:00 for about 80-85db.
The CAT allows for on the fly loading via resistors mounted on RCA plugs that connect just like interconnects. I most often use 300ohms with the ZYX, while I chose to load the Shelter at 100ohms or 150ohms.

Frank, anytime. You might want to call Ken and discuss with him yourself. He is quite approachable. Cheers,
Spencer

Frank,
It's hard to say without having heard those combinations.
When I owned BAT amp & preamp, I used Balanced connections, because they recommended strongly doing do, and you need adapters to use RCAs.
My experience in general is that balanced makes a big difference only with VERY long cable runs(i.e. >25ft)are used.
Every balanced piece I've seen except the BAT also has single ended, so the designers probably think it's reasonable to use with single-ended gear.
I don't think there is a concrete answer to your question. Cheers,
Spencer