wht is the difference between good and bad sound ?

Nobody knows.

i think you can make a case that as the sound of your stereo system gets closer to the natural timbre of instruments, sound quality improves.

who would not want to turn on a stereo, close your eyes and have the illusion that you are present at a live event ?

Good sound is your system. Bad sound is someone else's system.

Why do we need good sound? The good sound makes people happy. The good sound will make you and your girlfriend romantic! It makes people very easy to get close each other. You fall asleep on music easily and wake up refreshed when you are tired. The good sound is something like a beautiful singer is singing only for you right in front of you, and the song touches your heart. Also, you can see and hear (in your imagination) singer’s watery and long lips and soft and strong tongue hit each other and saliva splashes in sound details. When you listen an orchestral music, you can hear and see specific location of many musical instruments as if you are in the main seat of concert hall.

If you have a good sounding system, you will not eager to come home to hear your sound system and crave for music because you are satisfied in sound and music. If you are not satisfied with your sound system, you will have upgrade bugs. If you are analytic about your system sound and keeping you from listening and enjoying music, it’s not really a good sound system. There is something wrong with the sound even if the sound is beautiful.

There is no listener's fatigue with the good sound. I get easily tired when the sound is thin because I (my brain) try to hard to catch/listen the sound. The good sound will come to you. You don't need to concentrate or anxious to listen the sound. With the good sound you will just relax without much effort to catch the sound. Thin sound isn't easy for your brain. it's opposite. Thin sound is OK for background music at the bookstore or market.

Thin sound can't be match with a powered sub-woofer. The main speakers' sound will wash off easily with the powered-sub. Naturally, high and mid freq. stand out better with a powered-sub. Many audiophiles don't want the powered sub. because they want cleaner (but thin) sound. Without sufficient low freq., people will never musically or sonically satisfied. Again, if you are not satisfied, you has a upgrade bugs.

If main LR speakers are normal phase, the phase of powered-sub will be 180 degree. I don't want powerful sound waves hit me directly. Let super low freq. waves come from everywhere. This way I can set the volume of powered-sub much higher. It's much more enjoyable.

Your assessment is an interesting one in that your description of 'thicker and vivid' as opposed to 'thin' is my observation with the Acoustat TNT200's I'm using. But I'm referring to the ability to 'dial in' the sound I prefer with the bias setting. These are mosfet ss amps whose setting is a stated 300ma per ch.. I run them at about 291 ma. Less is 'thin' sounding, more is too rich wherein the leading edge of transients become softened and slightly obscured. I had no idea I could tailor the sound this way until I recently stumbled on it reading a thread on DIY Audio. Excellent new tool to play with:)