I use the MB210 for my louder gigs and I've always had plenty volume / headroom.
The combo is bass heavy, I often have the bass control around 1/ 4 - 1/3
I never have the master past 1 0clock
The Mitre is a small venue, I would normally take the MB200 head and a single UL110 to a venue that size.
All I can think is the rest of the band are too loud.
If they want to play authentic sounding rock & roll with double bass they should cut the volume to suit.
Who was on drums? Animal?
P.s.
Don't forget the phase switch, can work wonders if you are close to the speakers, which I guess you were at the mitre.