He has already installed a good quality speakers (being JL Audio). Dynamic of sound is defined as the ratio of loudest and quietest sections for a given track. Good external amplifier gives REAL power across the complete frequency range, not the crap from selling material stating that each HU is pumping out 4x25W as this is momentarly peak power, usually measured at 1 kHz signal for a brief moment.
Also, a good amp will have a very good damping; driver on speaker has it's inertia and after it was moved out of it's initial position it will want to continue moving and will move. That will in the coil driving it induce unwanted current flowing back to amp, or distorting the signal.
Please, note that I haven't said a thing about subwoofer, and if you go through my install (the link is in the post) you will notice I don't have a SW either. If you want a full sound, with strong and DEFINED bass (again, nothing to do with SW!) you need a good amp driving the speakers. The amplifier I am using (Rockford Fosgate 4.6x) is rated at 30W/channel, so, judging just by the specifications of HU not a big improvement over 4x25W, but in reality there is no comparison between the two.
So, to conclude: good quality amplifier will always improve the sound of any system, just by virtue of better control over speakers' membrane movements and adding that extra power for getting to the bottom end of what speakers can produce.
Again, I am talking about SQ here, not chasing dB numbers
