What we hope for in any piece of hi-fi equipment is sonic transparency. We get there by practicing the Hippocratic oath, "First, do no harm."
And while this is all well and good the facts are unfortunately not in our favor. All electronics affect the signal in some way: robbing or adding. There's no such thing as harmless.
What we practitioners of the art can strive for is to leave as small a musically agreeable footprint as possible—to do no harm, and what little impact we have should be toe-tapping good.
At least when transparency's the goal.