good sound quality

I own 1997 honda civic si and planning to instal a good sound system. I want good sound quality. I did couple of researches and readings but until know I still confused. I can't decide because so many manufactures make a good products.

How can I choose a good amp? I readed that if you looking for a good amps you need to make sure that THD is .001 or lower. Is this for all class amps or not includs D class amp? Are THD and RMS numbers the only factors to judge if the amp is good? Not many companies telling THD number on their products spec sheet, like Rockford Fosgate they use IM distortion (IHF). What is the diffrent between THD and IM distortion?

What things I need to pay attention when I'm looking for speakers and subs? Do I only need to pay attention to RMS, frequency respond, and sensitivity? My brother got 2 Perfect 12.1D, are they any good?

I hope anyone can help me out, thx.

MB quart is the best in sound quality.

I have a pair of older 12" from them and they are really nice.  also i hear there speakers and components are sweet.

good sound Q cost money get your self good componet sets, head unit, amps, & subs. for SQ dont be scared to power the dog s**t out of them. with a good head unit and amp they wont play the low tones to hurt them. next  sub(s) look for a true sub and a class d amp to drive it. that is the start for great SQ.