I spent some time tonight with my M6, MB200, and the STM900 through the Berg AE212. Since the M6 has been my go to for a long while, I tried to dial both the STM900 and MB200 to similiar to the M6. I used an ash/maple JO5 and then after I got the sound in the ballpark I used some other basses. My first gig with the STM900 was a bit of a struggle, but after playing around with it and the M6, I now know what I was missing. It was some upper mid presence and treble. The settings shown for the MB200 and the STM900 will get you in the same tonal space as the M6 set flat.
I really did like the setting with the gain in, however with some playing around I am really fond of the gain out, gain around eleven and the input volume around 3 oclock. Running the input volume this hot does really open things up for me and then bumping the mids at 2.5 and bumping the treble gets me in my J bass territory. Some of you won't like the extra clank and treble of this setting, but for me this is the presence and clarity I need. Can't wait to try it at Sundays gig. Feeling better about this amp, and I believe as Jason said it takes some playing around to really know how the tone is affected by each control.