invalid, that is precisely the comparisons I have done, not only with an amp, but with several components together! Read my articles "Audiophile Law: Thou Shalt Not Overemphasize Burn In", and "Burn In Test Redux" at Dagogo.com, and you will see that even multiple components and tweaks were unable to cause change to the sound quality.
I subjected all these items to what I call the Imbalanced System Test, wherein IF there is a significant change, there will be damage done to the soundstage, tonality, center image, dynamics, etc. Example; I recently had a niggling issue with the sound of a speaker system that influenced the entire presentation. I discovered that one of the small discs I place under the rear footer was not in place. Once put in place, the presentation was normal. Similarly, if there was, as claimed by so many�, significant improvement in sound quality, then when multiple products that are broken/burned in are used alongside the other channel where they are not, there should be a noticeable, significant difference resulting in poor sound quality.
There was not, which leads me to the conclusion that all the machinations and concern about break in is a waste of life. :)