^This 100%. Keep the fade at 0, you'll lose all volume if you set your fade to the back and you'll lose your highs. This radio is set up for highs and vocals up front, mids in the doors and all your bass from the rear deck. I find the volume at 21 or higher quite loud (depending on the music) so I usually have it at 14-19 when I'm on my own (I value my hearing).If your passenger rear door speaker is rattling then you must have it faded to the rear. The whole system will sound very weak if you fade to the rear. The Pioneer system is setup with a front sound stage, and all of the sound is only supposed to come from the front. Put the fader in the middle. If it's in the middle then you won't hear the rear door speakers at all, even if you sit in the back seats.
If the sub woofers in the back deck lid are rattling then something is probably loose. When I got my car the buttons on the back of the headliner weren't pushed in all the way, and the headliner rattled and vibrated a lot with sub bass notes. Sit in the back seat and see where the rattle is coming from and check if anything is loose. If the speaker itself is rattling then you should take it to the dealer and have them replace the speaker.
The Pioneer system is not a high power system, but if it's tuned right it's more than loud enough for the average person. It'll make your ears ring.