Sound System Questions, 5 quick questions :) Options

[JonCars17]

I have a few sound system questions.

I'm thinking of adding a sound system to my car by the end of May which gives me enough time to save up.

Q1:
What is the point in putting a Sub (8") in both side doors?

Q2:
How much should I roughly look at spending for the system (components in the front, 2 speakers in the back, headunit and a sub 10" with an amp)? I'd like to get one for qaulity and not the extreme bass.

Q3:
What should I look at when choosing a system? the Sensivity aka the dB?

Q4:
How do I calculate how much amp I need

Q5:
What can I do to stop the car from rattling once it is installed?


I'm a n00b when dealing with cars -_-.

[yyet]

Okay, if you're going to use the Alpine SPR-17LS, you need a separate amp for those. I have those component speakers in my fronts too and they only sound good off an amp, not just the head unit. And you don't really need Infinity back speakers, this is your chance to save money. Back speakers are mainly to fill out your car. But i didn't know that before I bought back speakers. In my backs I have the SPR-17LP (Alpine Coaxial Speakers). But if you truly want those pairs of fronts and backs, then you need a 4 chan amp that puts out around 75W RMS @ 4Ohms per chan.

I got SPR-17LS Fronts SPR-17LP backs running off MTX-MXA6004 amp (4 chan 70W RMS @4 ohms). Sounds Crazy...

Good luck...
Yet