guitar heads and distortion pedals

[a e i o u]

Does a distortion pedal mask the amp's true tone? I don't totaly understand this. For example someone will say that their Mesa Tripple Rec sounds awesome but when they play, they use it on the clean channel and with a distortion pedal. Wouldn't the tone only be as good as what the distortion pedal will do? It wouldn't make sence to spend two grand on a head and then use a 30 dollar distortion pedal when it seems as though you could get a similiar tone with a cheap head and a good pedal?? Maybe i'm way out of line here but i can't figure it out. Do you think more people with really nice amps use the amp's on board distortion?

[CrazyArcher]

Any tube amp has a certain amount of OD even on clean channel, especially if you play loud, so it definately adds a touch to the sound. The question is how much, so well - I don't understand playing through Mesa and MetalZone too...