EMG's are low-output (low impedance too?) into an on-board pre-amp.
Passives are regular-to-high output
Both have different frequency responses, you're not going to get the same sound by just adding a pre-amp to the passive pick-up. Part of that is due to the compression that's part of the pre-amp in the EMGs. You might get closer to what you're looking for with a boost into a compressor, or a compressor with a built-in boost/pre-gain.
EDIT: This is the image I was looking for:
Look at the dark orange and light blue lines. That's the frequency response of two EMGs. Very heavy on the Mids with a gentle roll-off on the bass and treble. The other graphs are the frequency response of typical passive pick-ups -- Basically flat until the resonant peak, and then a sharp drop.
If it was as simple as a pre-amp, then these:
would sound like EMGs. Those are Joel De Guzman's CycFi XR Pick-Ups
and they're active, like EMGs, but are not compressed and have a huge (~ 20 to 20k), nearly flat, frequency response. EDIT: Look at the darker blue line in the graph above.