The blue trimmer connected to the grey, blue, and white (or green) wires is for adjusting overall gain.  The response of the gain adjustment is flat.  I generally recommend setting it as low as possible.

When set to 0 (all the way counterclockwise), the output level matches the blend-loaded output of the pickups (gain = 0dB).  When set to 10, the ouput is 4 times that of the blend-loaded pickups (gain=12dB).

This adjustment gets used in a few different ways.

Most common is to use it to balance ouput between several instruments.  The strategy here is to set the gain as low as possible on all instruments and adjust up to match the one with the highest output.  This means though that the passive mode ends up being used as "emergency" mode - or, "Doh, I forgot to change my battery when I changed my strings!" mode.

The other common way to use the gain is as a front end to a level sensitive effect like distortion.  Here, the gain is usually set relatively high to ensure that it will always provide the ouput needed to drive the effect in a predictable way.  Passive mode is generally just not used in this case.

If you want to balance active and passive, you just want to set it to 0.  Of course, when you have any of the EQ bands boosted or cut, the passive level will not match.  Also, when in passive mode, the pickups see the load of the volume pot and everything in your signal chain, so depending on the impedance of your cable, effects, bypasses, and amp, your signal may end up quite attenuated.