wht is the difference between good and bad sound ?

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

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:)

The closer the sound coming out is to the same sound in its natural, pre-recorded state, the better it is.

That's why they called it high fidelity.

I think that's a bit wishful if you're suggesting that's possible with play-back gear. Hi-Fi is all about the play-back gear. But I do agree with your initial statement.