i've played a couple of open air gigs on large stages with the same amp as you and I two Markbass 2 x 10"'s. Didn't go through the P.A. Infact it was all pretty much our standard gig set up, ( can't recall what the P.A. was)
Anyway volume wasn't a problem.
The problem I encountered was with my amp cutting out. It was a momentary thing but happened a number of times during the gig. I thought at first that the amp was playing up but realised after a while that it was something to do with the generator.
The amp has never missed a beat on any other occassion, indoors or out, just that one gig with the generator.
Sorry can't tell you why it happened, although I suspect the supply was just not consistent enough. I suppose the advice would be look for a high spec generator, if there is such a beast.