Freak,
> Can any one tell me what the main QUALITY differences there are in passive and active monitors. <
Assuming otherwise equal speaker driver components, there are many advantages of active monitors for the typical project studio, plus a simpler hookup with less pieces to carry if you do remote sessions:
Active speakers are typically biamped, which often yields a cleaner sound with less distortion. And biamping offers more ways to optimize the crossover performance because it uses active rather than passive components. Also, the power amps will be well matched to the speakers, they won't have a fan, and the wires from amp to speaker are shorter which improves damping. But to me the overwhelming advantage, as implemented in the Mackies anyway, is that the woofer cone's motion can be included within the power amp's feedback loop to reduce distortion by a significant amount when compared to passive speakers with an external power amp.
--Ethan