That's not correct logic, exactly. You need to put the passives through the preamp. The actives are ALSO going through a preamp, but in the case of actives like EMGs (the specific brand mentioned in this thread), they're already going through an inbuilt preamp. You don't want to put them through a second preamp stage, and it certainly wouldn't match its output to passives.
So, you're kinda sort of understanding EMG's active technology, but you're missing a crucial step which is throwing off your understanding. The other step that you're missing and which I, too, am guilty of not making clear... is that it's not actually raw volume that you're trying to match (though, altered output level is an effect of preamplification, too) but impedance.