Your amp should have a line out, or DI out, or pre-amp out, or something, that the soundman uses to DI your bass from the pre-amp to the desk. If not, he needs a DI box which you put before your amp in the chain.
Unless he's micing your amp, he doesn't need to mess with the output volume on your amp. The output on the amp is YOUR level. The fader on the mixer is HIS level.
Not sure if i've explained that properly but yeh, i'd never run an amp head at max volume.