First off, let me say I am blessed to have a tech who is not only talented but also a true enthusiast. Everytime I am at his shop I'll bring over an amp or receiver for him to -*test*-('") with his function generator. He will input a square wave at 20 Hz, various midranges, and at 20 kHz and 50 kHz (ultra-sonic, for laughs) to see how accurate my amp is. I must say, this is the most objective measure of sound quality I have seen (you old tech hands have seen a lot more so bear with me...).
Generally, the better the result, the better the sound. I'm getting to the point where I can correlate the sound quality based on the square wave trace. I am also getting to the point where I only go by the square wave result and not the subjective OMG, OMG opinions we read on the various boards. Call me a snob but I am a believer in the square wave -*test*-('").