Just no crystal in this clear...
Here is another way I think of it.
Think of the experience with driving the power section of an all in one amp with an external preamp through the effects return. If there is no 'post make-up volume' control, often one preamp will sound loud and another will hardly drive any volume at all, since the sensitivity of the power amp input is matched optimally with the internal preamp.
So, the internal Minnie in the DeMonique is like the internal amp in any 'all in one head'. The gain from the preamp is optimally matched to the preamp, hence, one 'master volume' control that just impacts the preamp level hitting the power amp after all the other preamp circuitry.
This is the situation with the 'level' control on the DeMonique.... it is equivalent to the master volume control on any amp, and can drive the internal Minnie to full power with optimum signal to noise, etc.
That ADDITIONAL master volume control on the 'stand alone Minnie' is basically a gain matching control for preamps that aren't specifically optimally matched to the input gain/impedance (whatever) of the Minnie. So, you might have to crank it or turn it way down with another pre to achieve the same optimal matching as the internal DeMonique.
On the other side of things, that is also why the Monique has that 'push/pull' boost setting on the level control.... to help it work with amps that are not optimally configured to work with it.
As Jule says, you can't really hurt anything by turning the knobs this way and that with the component Monique/Minnie rig, but at the extremes, it will add either noise (Minnie volume set high) or distortion (Minnie volume set too low).
The only thing that makes this 'unique' versus other PA amp/preamp component rigs is that the Minnie's input is optimally matched to the Monique, since Jule and James got together. So, a unique situation, and IMO why the Minnie sounds so amazing with the Monique! The DeMonique just sets the Minnie gain level optimally for you.