Why go with active preamp?

It's real easy...you buy a good active preamp and compare...play a variety of music with both set up's, including dynamic stuff...then you'll know.

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.