I think the one issue not addressed in responses is controlling [driver] phase to create a phase linear system. Phase control is not possible in the passive or the outboard amp active system (unless you have a very sophisticated line level crossover with phase controls on each band, which I have not seen outside of DSP crossovers). It always requires careful measurement to be able to accurately adjust phase. Therefore, phase linearity is one attribute of an active system that is hard to compete with.
Don't get me wrong, I'm not against passive, its not shite, it can sound very very good indeed, It's just that active has so many positives that seem so poorly understood in the audiophile community. It's like there is this feeling of "I'll give you my outboard amp when you pry it from my cold dead fingers!" It's a little odd when the science on this is not new or controversial and the sonics are obvious to anyone whose has tried like for like.
The anecdotal evidence I read in this forum doesn't really compare active vs passive, like for like. Its usually comparison one brand to another with other differences that are not explained. Granted a proper demo is extremely difficult to pull off, maybe impossible. I've done it because I have the same speakers in active and passive and the same amps built by the same company. Not everyone hears the difference, like my wife, so tis not black and white. But it certainly is as significant a difference as one cable vs another., one CD player to another, one DAC to another. Maybe sometime I'll invite you all to Las Vegas to hear this comparison. Or maybe we'll do this demo at AXPONA next time and have a party doing it!.
Brad