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,
I just wanted to post here if someone comes along who seeks this information.  I know the original post is old, but this information is useful nonetheless :)

All tubes are ECC83 / 12AX7 type.

In the ATC-1 there are three tubes on the left of the chassis. Those are the phono tubes. The middle tube needs to have balanced section and can be whatever brand you want to try.

Outer tubes of the three need to be a matched pair. I would try and get the quietest tubes in the phono section possible.

The tubes in the line stage section of the chassis need to be a matched pair.

I have found a fantastic sound using the following tube complement right now:

1) Phono Section: Telefunken ECC83 Smooth Plate (NOS), Matched EI 12AX7 Gold Smooth Plates (Modern production tube made using Telefunken tooling).

2) Line Section: Matched Pair Mullard 12AX7 Blackburn Factory.

Sound is detailed, smooth and seductive at the same time. Playing through the Air Tight ATM 1 (Tube Rectified Power Supply, not the current Solid State version). Superb combination. My sincerest respects to master designer Atasushi Miura San. I feel so much gratitude...