Sound Quality is judged in many subjective categories...
You have two judges get in your car and they take over...you just stand outside and watch. They listed to special tracks of the IASCA disc and they look for staging, depth, transitions, balance, center fill, ambience...the overall sound stage that is presented.
In other words, is the guitar where it should be? How 'bout the lead singer? Are the drums centered and placed behind the band (remember, from the drivers or passengers seat, the drums will actually appear out on the hood!)
So, you are judged in these categories on scales of 1-10 and the like...