What would cause this? Pre question

[davec]

I have a bass with on old Hazlab pre and Lakland P/J with a flipped coil pup's, new battery.
When the J is soloed the volume with the pre is much softer than running passive?

 

[PSPookie]

Doesn't engaging the pre lower the impedance? That could cause a drop in volume as well as a shift in the resonant peak. If this is a significant enough change and the preamp gain isn't high enough to compensate then you could get a reduction in overall volume.

I think