Speakers, amps, and subs can definitely make your cabin louder, but, their is a lot of ambient noise that gets into our cars from that plastic door coverings you can see in Radioguy's photos. I was on a stretch of hwy and pulled out my RTA (sound monitoring equipment) and recorded the db's just driving down the road. First pic.
I installed Dynamat on both doors and the difference is amazing! Much less ambient noise. I will post a new RTA soon. Blocking the white noise coming in from the doors will make your interior music less muddied.
Also, Dynamat will increase the depth of any speaker system you put in the car. Allowing for a warmer tone quality because the attachment point (your car door) isn't just a metal plate.
Amazon has the door kit for around $50 which is about half of retail. Kind of a PAI install but you will be happy with it when you are done.
Originally I was going to go with a 5 channel amp. So far, I changed all speakers to the Infinity Reference speakers and am getting more punch than I originally thought because of the Dynamat on the doors. I will probably still want more down the road but am quite impressed with the Dynamat and 4 speaker set up.
Just my .02
2013 Black GT
JLT Cold Air ~ Bama Tune ~ SR Lowering Springs (1.4" Drop)
MMD Classic Quarter Panel Louvers ~ MMD Side Scoops
Kid seat in the back