Air Tight ATC-1 tube questions

Got the itch to roll some tubes in my ATC-1 preamp.......more specifically, the phono section.

This pre has been around since early 90's but not the most common piece of gear out there, and I'm having trouble finding info and user experiences on it.

Anybody know how this MM phono circuit works?
It uses three (3) 12ax7 tubes, accessible on the outside of the left side panel.

Specifically, my questions are:

(1) do all 3 tubes need to be identical and matched?

(2) or is a pair required? If so, what positions?

(3) or can 3 different tubes be used? If so, I guess this means the triodes are being split in each of the tubes to obtain L and R signal.

(4) if singles or pairs are OK, which position(s) has the greatest sonic effect?

(5) can 5751 types be used?

(6) can 5751 be mixed in with 12ax7?

Also, any input on what tubes you liked and why is greatly appreciated.

Thanks in advance,