Which tube is the 1st gain stage? How do I know so I can try a different tube?

Generally closest one to inputs.

 

There is quiet a bit of variation in gain and tone between various 12AX7 types. ECC83, ECC803, 7025 are all basically names for common 12AX7 types. If you want slightly lower gain you can also try 5751.

 

Can you tell us the reason why you want to change (on your own) the initial stage of the amplifier?

edit, guitar preamps initial stage are sometimes rather less cathode biased which may make tube rolling noticeable (at some content).
Bass guitar (and all purpose) amplifiers are most often "standard" biased at the initial stage which may make tube rolling like an esoteric science.

 

http://traynoramps.com/downloads/ownersman/om_yba-1.pdf

 

The cathode resistors for V1 and V2 equals 820 Ohm which predicts a rather highish (total) gain.
Furthermore the vol.-pot is directly (AC coupled) wired to the anode resistor of the initial stage which then (also) improves the total gain structure.
Obviously the preamp schematic was designed with respect to generate some distortion.

I don't think that tube rolling alone will bring you to the desired gain characteristica.
It might be beneficial to mod the schematic a little bit just to meet your personal taste and needs.
This mod could be easily done by an experienced service tech.
Modifications should be done with care and respect to provide the possibility to also bring the amplifier back to original specs