Active moniters are a bit different from just supplying your own power amp to passive moniters. Mainly an active moniter has its crossover splitting the high and low frequency of your signal BEFORE it gets amplified. It then feeds TWO seperate power amps within the same box so both the woofer and tweeter get thier own individual powered signal at thier ideal wattage and respective frequency. The advantage of this is minimal signal loss compared to passive moniters which use a passive resistance type crossover that splits the powerd signal AFTER the power amp.
As a result, active moniters tend to have a more pure sound with better presence and clarity over passives which is why people like them. Thats not to say quality passives cant also sound great too, but when you start to add up all the extra components and features that go into active moniters, you begin to see what justifies thier higher price tag. To duplicate the same thing with passive moniters you would need to use an active external crossover with two seperate stereo power amps yhen modify your moniters to receive both hi and low inputs.
Although I'd definately agree with that when it comes to high powered PA stuff, I wouldnt nessisarily apply that same logic when shopping studio moniters for the reasons I stated above. On a tight budget however, good passives with a cheap amp might very well work out better than any bottom of the barrel actives in the same price range.
