Each mod version is presumably better. How much better is impossible to say, but even the base model is a very nice preamp.. 6922 is a 6dj8 variation and said to be a better version—at least in theory, but its easy to silkscreen anything on a tube, so I would not put too much weight on that. Either tube works. What ARC did at any given time may have just reflected their part sourcing situation.
Audio Research SP9 family
I want to know more about the Audio Research Family. What are the differences among Mk. I, II and III? I learn that the SP-9 family uses only 2 6922- does it mean that the SP9 use tubes in the phono stage and transistors in the line stage (like SP14), or one tube for the phono and one tube for the line? How well was the SP9 accepted when it was new? How does it sound? Is it a good value?