What are good models and manufacturer's for actives?
Genelec 8050A - you may find these at your local music or guitar shop - they will probably be close to the best of the active near fields mini-monitors you can audition at a pro music store (where they sell musical instruments an studio gear)- try to listen and see if you like them - I think they are better than Mackie's - if you do then there are a few others I would add which are difficult to find for an audition and even more expensive and will essentially play louder for far-field listening - so it is probably best if you check these Genelec's out first and see if it up your street. (You may not like the neutral sound -many don't)
What are the downside to actives and why do are there so few active's as opposed to passives?
This has been discussed on other threads but basically you are limited for "tweaking" through choice of your own amp - especially important to tube lovers - as you won't find a tube in an active speaker (due to microphonics of tubes)