Pedals tend to be made more for people who just want distortion to practice or play around with. If you're looking for a solid professional edge you probably won't find it in any pedal, but you will in a better setup.

Besides, if you have anything like a Mark IV or a Marshall Mode 4 (cheers for Static-X) odds are a distortion pedal isn't going to do anything better than what you already have. It's not so much a matter of messing with the amp's unique sound as much as it's just using something that sounds worse. That isn't to say awesome pedals don't exist, I just haven't ran into any myself. If I see any of the ones Groovemule mentioned I'll be sure to try them out, they could be good.

So, yeah. If you have a decent stack you probably won't be needing any stomp boxes for distortion.