This sounds very imteresting! I never knew this was possible!
I have 2 Fortresses which were fine until I got my custom FNA, which is the LOUDest bass ever! Seriously, the output is monstrous, and when I switch the FOrtresses sound wimpy and tame.
I am now going to try and boost them a little. They don't have a passive mode (these are Masterman models) so it shouldn't matter much.
The level on the FNA is fine with the passive mode so I'll leave that alone, even though it causes clipping at low input gain.
OT: I used the passive mode for the first time recently on a gig when my battery died, and I was VERY impressed! I have been a snob about passive basses but I take it all back. I could hardly tell the difference!