I would say that compression and the crappy MP3 sources are hard at work here. OEM equalization used to be tuned to the car interior, the newer stuff certainly has max volume levels way below what they should have; telling me they compress and limit dynamics.
This is overcome by summing the channels together and running the amplifier with a higher input sensitivity setting.

At he end of the day, the power and output of OEM radios/amps is much superior than their speakers, which continue to be a total joke.
My '14 FX4 came with the Sony 700W upgrade which is better than standard, but the electronic know it all's of compression and contouring really make for a poor experience. Play loud for a bit and it seems like the power level drops once the amps get hot. At low volumes, the sound quality is ok, but certainly not for the cost of the add-on plus the original radio - I'll be summing all 8 channels into a Cervin Vega Pre amplifier and then running a fancy electronic crossover (Audio Control or Pioneer CD1000!!!) and a few amps.....plus of course all new speakers.

The factory sub is a real piece of shit, too small a box with an 8", nut adds bass at low volume levels, especially with the windows open. That can be improved by adding some damping material inside, but I haven't gone there yet.

Something about a car I'm supposed to be building?