The thing about the GK head is that to me: while it is punchy, loud and articulate, and also very precise, it also seems dry to me tonally. It is not growly at all. I like a bit of that warm growl to my tone.
That's odd. GK heads are known for their growl. I haven't tried the MB Fusion series, but I've played pretty much every other head of theirs fairly extensively.
Assuming the MB Fusion series is similar enough to the MB series, did you use the B channel? That's the MB's verison of the RB series Boost.
Crank up the Boost and you get GK Growl. Crank up the Master while cranking up the Boost and the "hit the rails" on an RB head.
The MB series did a pretty good approximation of hitting the rails, but they don't have the same power supply so it's literally impossible to do it for real.
It's was good enough for me to use in a live environment. I retired my 84' 800RB from daily gigging and bought an MB500 to replace it.
To get that amount of growl out of a Monique you'd need to run a boost or OD pedal into it IMO/IME. The Sensitivity Knob will give you some nice harmonic content, but even cranked and pulled out it's not enough for me (coming from GK and Ampeg).
Of course, your bass and strings comes into play as well.
After all that, growl has different meaning for different people. When I think growl I'm thinking Geddy Lee, Duff McKagan (GK), Mike Starr (Ampeg), Justin Chancellor (GK), Rex Brown (Ampeg), etc.