It used to be on their site, but Bart no longer lists the dual EQ preamp that was used in the original Growler. However, the
Zon Sonus Studio still uses it (or a variation of it)--Bartolini may make it exclusively for Zon now. They call it the "ZB6", and I'm pretty sure that was not the original name. You might contact Zon and see if they can get you a wiring diagram, or you might try and contact Batolini and ask them about the "ZB6".
That would be for wiring digrams showing you how to hook the preamp up, of course.
I suspect the current Growler just uses a standard Bart NTMB 3-band, with the two halves of the pickup going to the blend as iff they were separate pickups.
Mike