well, here's the diagram from the bartolini website. http://www.bartolini.net/instruction...b/ns2tmb18.htm
From the diagram, and the title, looks like this preamp has a buffered input stage so that none of the usual issues with passive blending happen. Also lets you change between active and passive pickups without having to change any pots.
The U/J-Retro preamps from John East and, I believe, the Demeter on board preamp also have this feature.
With this, there's no easy way to incorporate an active/passive bypass.
Someone correct me if I'm wrong, but the trim pot affects it at the input stage. The volume control affects it at the output stage.