the same question was just addressed
here.
the "systems" include volume and blend controls which will
not work with passive pickups.
the EMG "controls" are the preamps from the systems by themselves, and they
will work with passive pickups. (they work
great with passives, actually; the EQ is smooth and tasteful, and when i've wired them with bypass switches to go back to straight passive, the pickups sound just about the same passive as they do active with the controls flat.)