preamp tubes definitely do more toneshaping that you immediately hear, but the overall charactaristic can be heard from the powertubes. the way i've heard the common types is pretty general but goes like this(and these are only the ones i've tried):
6l6:rounder with bigger lows and smoother highs, tighter distortion, higher definition(fenders, mesa boogies,5150s, etc)
el34:raunchy, fatter mids and more grit, even when clean(marshalls)
el84:chimey, lots of mids and treble, not nearly as much bass as either 6l6 or el34s(vox ACs, egnater rebel)
6v6:fat, chunky, kinda like el34s with less top end.(fender champ, egnater rebel)
this is totally a sweeping generalization, but you get the picture.
