Let's all get one thing clear real fast... NO ONE except the engineer and anyone involved in the mixing of the final product KNOWS what it should sound like. The bottom line is that we're just tailoring our systems to:1) What we think it should sound like
2) What we want it to sound like
3) What a judge or some other organizations tells us it should sound like
My definition of a good SQ system is one that I can sit in, throw a lot of various music in the CD player, put in drive and enjoy the whole way to my destination without ever feeling like "God, I've got to fix that later" or "could use more X". Until I get to that point, I'm still working. I recently heard a few cars that wouldn't win any world championships by any stretch, but I'd MUCH rather have those as my daily driver than the ones that would.
Cliffs: It's subjective, for the most part.
For the most part, yes, it's subjective, but...
There are boundries that are outside of "subjective". There is objective good and bad "SQ".
That is why "refrence" material is so important. A recording of a chello solo, should sound like a chello. A piano solo should sound like a piano. Of course pianos can sound different, especially when recorded in different ways in different rooms. High quality refrence recordings reveal flaws in even the best systems and rooms. That is why we use them to test systems.
George Winston at Carnigee Hall should sound like a great piano in a nice room. Yo-Yo ma in the studio should sound like a great chello sitting in front of you. Etta James should sound like she's singing in your face. If it's boomy, tinny or muddy, it's your system.
Steely Dan is a popular choice for sound system checks, not because we like Steely Dan, but because his recordings are very high quality with clean clear instruments (especially drums).
If you can make these kinds of recordings sound good on your system, with minor variation, then your favorite tracks of Slayer or Tupac will jump out at you with realism and clarity you didn't know was there.
A little bright or bass heavy can be subjective taste, but clarity is clarity.