The tube preamp in the MP-2 is control-wise build very differently than that in the MP-1
The MP-1 pre is quite a simple design, where each tube simply has it's own gain control.
The MP-2 preamp is a different design, which has a lot of control/setup voltages (these are handled by the vactrols MJMP mentioned).
So, instead of handling a relatively simple 'each tube it's own control' scenario, ADA chose to implement ten differently sounding voicings, each of which will setup the two tubes using the control voltages I referred, and you, the user, will use a single gain control to affect all of this in a rather smart way, methinks.
The Master Level control is simply a.. well.. master level control, positioned after the whole tube assembly.
In other words, the gain control and master control have no 1-to-1 interrelation; however, when you turn up the gain, the signal level will of course rise, so master needs to trimmed. But there's no interaction, like master inflicting pain on the gain.