Re: Distortion first or distortion later?????
Depends on the tone you're trying to achieve. I love the distortion I get out of my Marshall AVT50 Head and 412 Cab so much that I practically gave away all my old distortion pedals (the clean channel, however... leaves a lot to be desired for).
Different pedals produce different tones. Compare a Tube Screamer to a Boss Metal Zone... see what I mean? It all depends on the genre of music you are playing. If you're playing Punk, Metal, or most Rock, you may want to use the amps built in distortion with a slight OD pedal to push it a little farther. But if you're playing Blues or Jazz, get an amp with an amazing clean sound and just use a stompbox for your distortion.