Re. the ARC preamps - yes, I'm finding out that the SP-10 & SP-10 Mk2 were some of the finest ARC SP series preamps ever made.
Another data point from my perspective - I had the opportunity of hearing a ARC Ref 1 (connected to a Symphonic Line Kraft 250, Newform Research speakers, LP12, ARC PH-1 phono). That sound of that system did not impress me at all! It seemed like the sound was veiled & wanted to break free but somehow was trapped & couldn't! I felt that the SP-6A was much better. Quite remarkable since it cost much, much less than a Ref 1.
I understand that the Ref1 & Ref2 are hybrid tube designs like the SP-9. So much for hybrid designs if not executed just so!!
My speakers are a bit on the dark side so the accurate CAT preamp works great with them. My friend's are much more neutral & an ARC-type pre works great in his system. If I had the SP-6A in my system, it would be a bit to "syrupy" for me. I'll have to find someone w/ a SP-10 & compare that to my CAT.