Depends on how you use them. If you are running the dirt pedal into a fairly flat, clean amp you've got a million options since the amp will mostly preserve the character of the pedal. On the other hand, if you are using the dirt pedal to accentuate or alter the way the amp itself breaks up, the combinations chosen get much more interactive. The gain level and EQ curve of different dirt pedals can have drastically different effects on how the amp itself responds. Something like Treble booster into a very bright amp can sound like it's raining ice picks, however the same treble booster into a very warm and fat amp, say a Plexi normal channel, can sound amazing. Same goes for the midrange hump of Tube Screamer type pedals. Into a mid heavy amp, it can sound honky and terrible, but into a boomy scooped amp like a Mesa Rectifier, it can tighten things up and sound awesome. It's a lot of trial and error for the most part.
