What is the One Test Track That Tells You Almost Everything About A System?
There’s no such thing, of course.
But that doesn’t diminish the fact that which test tracks you bring to an audition are crucial to the productivity and success of said audition. Put another way, if you show up empty-handed and let shop staff spin tracks for you, you’re doomed.
Forget audiophile music: it sounds good on everything. Diana Krall sounds good on tailgate speakers. Dealers love audiophile music because it helps sell gear.
Do bring tracks that reflect the extremes of your listening tastes and habits, because ultimately you want a system to possess the range and ability to play both (and everything in between) to your delight.
Maybe I would play a Bert Kaempfert track at 65 dB and a Uniform and The Body track at 110 dB. A system that excels at both is more likely than not to excel at playing all the music I’m likely to ever throw at it.

