While it is customary often to use a passive bass with the volume at max, the retro has a lot of tonal variation, and a 9V battery doesn’t give enough headroom to take care of the high signal levels it’s possible to create, with the volume always set at max. But it will work no problem, if you follow as instructed below. Think of the Retro as a mini channel strip you find on sound mixers, where you rarely see the faders set at maximum. The Retro volume is similar to a channel fader.
Go to passive mode with volume at max, with both pickups selected, and set the volume of the rig to a good level.
Back off the volume and then switch to active.
Set the EQ and adjust the volume to taste.
Backing off the volume effectively creates more headroom.
Note: The above applies to all Retro types, including U-Retros.