You're probably going to get a few different opinions about how to set it up.
For my rig (tube preamp into hpf/compressor into XLS1500) I do my gain staging by adjust each (from the start of the signal til the power amp) to the maximum while still being clean when I play my hardest. Then I set my "final" volume with the attenuator on the XLS. May not be the best, but it works for me.
When they are used for PA use they most likely set the attenuators on the amp to max and use the mixer to control the volume since the amps are often quite far from the mixer and the engineer needs to have control at the mixer.
Be careful running in bridged mode with the attenuators wide open.... That's an 800W cab and a 1500W amp. I would keep it in bridged mode, but just listen carefully for signs of distress unless you use a lot of distortion, in which case I recommend changing it back to stereo and just use one channel (525W) unless you need the extra volume / headroom.
Flatwould Flatwound club member #506
My fEARful build:talkbass.com/wiki/index.php/Oobly