Unsound is right, there are many threads on this. In a nutshell, if you have an excellent front end with sufficient output v, and you don't want to enhance (or degrade) what it offers than try a passive. If you want to hear what passive will do for your system (which should be nothing or next to nothing), buy a used Placette RVC ($5-650), it only has 1 in & 1 output; but it is among the best resistor based pre's and will easily sell if you don't like it.
Passive Preamps
I'm curious about passive preamps. Has anyone tried a passive preamp in their system?