Why go with active preamp?

I went down this road already for a while. I think there are probably ways to make passive preamps (or no preamp) work, but you really have to be willing to tweak your system and try lots of combinations out, and you have to have quality source and amp components that deal well with no pre. That being said, I could never make it work in my system, and the sound from my new active (Plinius 8200 mkII) is pure heaven compared to my experience with a passive setup.

Sometimes something seems like it should work but just doesn't.

There's a lot of good stuff that's been said here, from seasoned experience. You can pretty much pool what has all been said, and try to get an idea of what may or may not happen in your experiences. I can pretty much look at each post and concure with some point, or all points, and say the scanario's mentioned here would likely be similar to what you might find if you experiement yourself. The key is that you'll HAVE TO TRY YOURSELF TO FIND OUT!
An easy way to compare what a passive preamp would sound like in your sytem would be to just get a good CD/DVD player that sounds very good through the analog out's, and that posseses a "gain control" or "volume control" and connect it dirrectly to your amp!(perhaps some sources have different gain and impedance, so beware) You can compare this passive set up to an active preamp you might be considering, then you can not the differences. You might also try tube preamps and better active SS offerings to compare that as well.

A buddy of mine has a Berning ZH270 with Merlin VSM-M speakers. We listened direct to the Berning through a Bel Canto DAC 1.1 and also through his CAT pre. We both much preferred the sound through the CAT pre. Much more enjoyable to listen to. We didn't feel the sound lacked the detail, etc. It retained the detail but became more musical. That's not to say through the Berning was bad. Hardly. It sounded wonderful. Through the CAT it was more musical and enjoyable.

i have never heard a passive preamp / direct out that had sufficent bass and mid-bass response. the mid bass respsonse is very slow and puddling.

also passive is very lacking in dynamics.

take a listen to a passvie unit / direct out to a nice tube /hybrid preamp and there is no comparson.

Try a bat vk30, audible L1, AR ls2bmkII etc... the soundstage will also snap into focus.

these are decent pre-s that are less than $1000.

With a passive preamp direct you should be looking for an amp with an input sensitivity under 1 volt. I use
a Gamut D200, and a passive preamp with the D200 set at it's .775 volt input sensitivity, with 5 meters of interconnect cable between the preamp & amp. Having just tried a active preamp for comparisons, there's no lack of dynamics, resolution, bass or treble response. As Marakanetz points out the midrange presence is affected.
In my system,the active preamp moves the midrange forward
as the volume is increased. Allowing that the power amp/speaker and impedance interfaces are good, adjustable input sensitivity on the amp and variable gain controls on the preamp would allow a user to dial in the sound they want, but there aren't many products that offer that feature.