VAC Signature MK2a pre "phono tube questions"

Hi there I have a VAC pre with MC/MM, upon opening noticed there are two roles of 3 12AX7 tubes in each totaling 6.

Question;

1. Anyone have any experience tube rolling these and if so with what?

2. Are they three matched pairs or all matched?

3. They are a Chinese tube that came standard with the pre., anything special about them?
I have found on other gear which came with Chinese 12AX7 tubes they were nothing special and by substituting I found the sound was transformed, sounding allot better using other tubes.

4. Can you mix and match, say the first role Bugle Boys, second Tel for examaple or best to stay with one brand through out.

I have allot of really nice matched pairs but only that.

Any assistance would be appreciated.