I thought maybe it was me creating beats in Manager poorly but the default beats vary greatly in volume while playing live also. Sometime it just skips the snare drum and the bass drum is so low its hard to hear. Then is all comes back loud when it wants.
You can change the volume using the BBMO, the velocity option allows you to adjust the loudness of a selected instrument.
Additionally, I recommend checking if you have the latest version of the default content (2.1), which includes improvements for volume stability.
I set the volume in BBM to 127 for all beats, I do have the 2.1 firmware. I will check it again to be sure.