what you said (not quoted to save space)
if someone asks for opinions, they are open to suggestion and wants to know what to learn from the efforts of others. if that is the case, I prefer to give them the best chance for success. a single midrange speaker per side is optimal. best place to start. our ears are most sensitive in that range anyway. we are less sensitive to midbass frequencies, so we want a bump below 200Hz. many agree with me, you are alone in your approach.
I have 10 well defined speaker locations for the front stage of my car that can be configured in a number of ways. i use 6 at a time in a 3-way active configuration. i've added drivers and ran more than one per frequency range to experiment. was it louder? yes. was it unfocused and unnatural? you bet. would I want to listen to that every day? nope. i don't know anyone that would want that when listening to music is their goal. now, when people want loud noise - the more the merrier and they can follow your approach.
experimentation is great, and i'm all for people trying things out to see what they like and don't. when you have the money and time to waste - go for it. i've spent hundreds of hours experimenting, i'm sure you do as well. i'll try things that people ask about, just to have an informed opinion. if you want to just do it once, take a focused approach making music sound good.
over the years, i've mixed driver sizes, overlapped frequency ranges, added more drivers to be louder - etc. when i was 19 i thought it was fine. then i met some competitors, learned more about acoustics, and started paying more attention to tonality. it's not about concert hall acoustics in a car - granted, you can look at those concepts and adapt them to a car. i attempt to explain the phenomenons that people experience in a car though the eyes of an acoustics engineer. i have come to realize WHY i hear certain things in a car. so i do speak in acoustics terms.
i agree that you can make a PA-style setup be very loud in a car by filling it with speakers.
the OP wants to enjoy music at a higher volume. but they want to enjoy music - not hear noise.
FYI - windows down at 55 - i still have a sound stage, i still have good tonality. if anything, soundstage focus is improved by eliminating the dreadful reflecting plane by my ear. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
lastly, above 130dBA you don't hear music. you hear noise and you damage hearing. i prefer not to give people tinnitus.