Why will my components sound like crap if I run them off a JL 300/2 if I use the stock HU?

All I hear is that it will hender their performance, I was just wondering why.

 

It will sound like crap because you the signal you will be feading the amp has already been aplified (by a lousy amplifier to boot), then needs to be dropped back down again to a level the new amp can handle (adds more noise), and then amplified AGAIN (including all the noise you've added up until this pointand more noise added, and then adding more noise to it as well). IMO, upgrading your factory head unit should be the first thing you do (most bang for the buck). Even a $200 deck will sound way better than most factory systems (*note* I haven't listened to a z's stero yet)