You piqued my curiosity, so I went down to the garage and dug mine out. Hooked it up to a cabinet that I haven't used in ages - a Hartke HS410B. Here's what worked for me: turn off the bright and deep switches. Set the tone controls to straight-up 12:00. Adjust the gain to about 1:00 and turn up the master volume to suit. It sounded damn nice with a passive Fender Precision Special. Thick, chunky tone.
I may have to drag that over to one of my rehearsal spaces and leave it. It's too good just to sit there unused, and it's too ***king heavy to carry around regularly for gigs. The Hartke sounds glorious, but it also weighs nearly 90 lbs and has uncomfortable lift handles.
If you have the Mesa M9, the Aguilar is not going to be ANYWHERE near as loud. The flip side is that the Aguilar is going to be a lot easier to get a usable volume on unless you're running at crazy SPLs. I have the M6, love it...but it's a fine line between "ooh, that sounds nice!" and "OMG TURN IT DOWN NOW!"