Wow, Chris jumping in with a pro HX argument...
But I agree, I use different coffees on a daily base and even though I am fairly new to the business I find it quite easy to adjust my Butterfly's temperature for different roasts/blends.
Cannonfodder you know of Ken Fox's and Jim Schulman's
comparison Junior vibe vs. Junior rotary? Since I first read that thread on alt.coffee I've wondered how representative this comparison was and also whether or not it's applicable to the general Vibe vs Rotary disputes. (Now don't get me wrong I don't question their results, but I wonder if such a comparison e.g. on an E61 machine would result in different conclusions)
But I've pondered too about Chris' "Elusive Clarity", where such minimal changes - IIRC, that is - in the chain as omitting the filter seemingly caused major differences in taste. Makes one wonder if such a test with a vibe-machine would produce similar results...
My own interpretations of this effect always seem to end up with what happens in the brewhead. The pump (vibe as well as rotary) generates a more or less laminar flow of hot water, but as soon as this water enters the grouphead/shower the pressure drops and the flow becomes turbulent. This not only makes the extraction process less "linear" it also should cause - significant, IMO - temperature changes on a microscopic level.
To cut the long story short, my hypothesis is, that the turbulence in the grouphead have a somewhat detrimental effect on the taste - especially to what Chris refers to as "clarity" - of the espresso and that instead a laminar flow in the cake should be able to provide this. But on the other hand, this also would probably reduce or even minimize crema...