I used a passive Yamaha BB3000S into a Low Down 15" cab, probably 8ohms, but I could be wrong (bought it used from a friend who perhaps had the speaker installed at Low Down's recommendation).
I set all the knobs to 12oclock, then tweaked from there. I left Gain at 12oclock, and only got volume up to around 11:30, with a tiny bit of Low bump for the stage I was on, and it seemed plenty loud.
I also run it through my Bergantino 610 and with a Bass VI, parts-bass P/Thunderbird jazz, and it's enormous.
I'm not convinced the Gain knob is for distortion unless Crunch is engaged, that's something to consider. See if the Manual has any info on setting Gain versus Master volume, etc?