I don't have a pre-amp but I do wanna give my 2 cents, If I am not mistaken, the reason someone uses a pre-amp is to control the signals that comes out. Also to get a flat signal response from your system. With a RTA (real time analyzer), you can really tune to that level.
You have those 7 band, or 13 band graphic equalizer in the old days. They suppose to serve the same purpose. The good side with a parametric pre amp is, you can adjust the frequency where you choose to boost or cut the signal. You can also adjust the slope (narrow/wide). However, these days, high or even mid end HU might come with these feature already. One example is the Pioneer DEH-5750MP (I am not using this HU, but I do want one :) ).
I don't dare to explain more as I'm a newbie.
Please get more info from
www.bcae1.com and the jlaudio site. You should be able to understand the uses of pre-amps such as equalizer and crossover.
Hope this helps.