People who care about what audio products sound like can be divided into those who need to KNOW — scientifically — something sounds good and those who abide by the old Duke Ellington adage, "If it sounds good it IS good."
When it comes to evaluating audio products not even professional reviewers agree on whether there should be an objective or subjective standard. A couple of recent posts by well-respected audio wonks — who happen to be friends with each other — concisely lay out the arguments on both sides.
Some people believe science can tell them what sounds good. Brent Butterworth at Sound + Vision just published a really nice primer on sound measurement and why it matters. In his view the perfect speaker has "flat sound" meaning it reproduces tones at the same level across the across the spectrum of audible sound frequencies. In other words they output exactly what you put into them. (Note: Your headphones should NOT have flat response.) Using just a microphone and software you can test a product and know its quality. Flat speakers are better because they reproduce sound that is objectively true to a recording.
Fine, but there's no soul in that position — at least according to a post by Steve Guttenberg at The Audiophiliac. Speakers that sound good just freaking sound good. Guttenberg says he's heard plenty of sound systems designed by engineers with their newfangled measurement tools and been very disappointed with the results. Something that objectively measures up might sound terrible just as something can sound good because of its imperfections. Humans are a finicky bunch.