Use your ears. It's not difficult to tell if the sound is coming from the center of the vehicle, in front of you, or from the sides.
It doesn't define, but it states its acronym. Do you see an acronym for SQL in terms of audio?
No, it's not only about a centered stage. There are many more aspects to sound quality. Driver choice makes a difference as well, and so does placement. Sure, you can buy $10 speakers, but does that mean it will be able to sound as good as some other drivers just because you can get it centered? Nope.
Some other aspects of sound quality are how well your setup blends. You have your tweeters/horns, mid range, midbass, sub bass. One of those should not drone the other and overpower it. If you have heard a true SQ setup, you would know what I'm talking about. It's easier to listen and understand than it is to describe. Along with a centered stage, it's good for it to be deep and wide as well.