I had a similar set of Alpine's. The woofer is made by Vifa, in Demark. So close to your car's birthplace. Lol
I hope you don't mind me posting pictures.
They had good midbass, but we're kind of inefficient speakers as a trade off. I had to run 300 W per channel to them, before they livened up! Not that they were seeing that wattage all the time. BUT when the dynamic burst in the music called for the power, it was available to create depth in the midbass.
Why am I telling you all this? Because I personally think you are suffering from lack of dynamic power. If you're using all the power to get them loud there is nothing left for the dynamic peaks in the music. The VERY first speaker was created by rice and Edward in the 20s. The very first thing they said is this will require a large power amplifier with lots of dynamic range to make music. That's from the mouth of the men that invented it.
What's the difference between a 2 watt amplifier with a 20 dB dynamic range and 100 W amplifier with a 2 dB dynamic range? Nothing! Without that available dynamic power the EQ can't can do anything.
The second thing to point out is that you can chase the reflections with the EQ. All it will do is shift the peaks and valleys to different frequencies. You CANNOT EQ out a reflection.
Clarity in music comes from tonal balance across all frequencies, for the timbre of the music and instruments to sound correct. The hopes I had when suggested the YouTube videos and the instrument frequency graph is this. To realize the sounds that make drums and such have an edge are not in the low frequencies. The crunch of a snare is way up in the 4kHz range. Even the whack of the kick drum is around 1000Hz.
In all honesty frequencies between 63 and 250 Hz do not need to be that loud. But they ABSOLUTELY MUST be in time with the subwoofer and the mid range to be present in the sound. This is due to the fact that our ears do not pick up great changes in volume in that range, but rely more on timing. Above 250hz is all about matching levels, below is ALL about timing.
Back in the 80s there were many cars with speakers all over the place. Then testing equipment became readily available!!! Then everyone realized how inefficient things were set up, and what could be improved on. Not necessarily better speakers, but speakers specifically designed for each application. More precise mounting locations. Amplifiers set up to give dynamic range. So basically less of just everything and more of just the right stuff!! This should give you an understanding of where these guys are coming from. Infact one of the posters works for an acoustical engineering firm.
More pics of me chasing the midbass!! Take note there are one, yes one set of baby 3 inch speakers playing the upper frequencies. On very little power and mounted very precisely. Believe me, most of the guys here would love to have 5 1/2"s on the dash! Myself included.
So yeah I love the car!! Keep doing you!! Hope you can take something from this and apply it. If not now, maybe on a future car!!
You seem like a humble guy that is into learning and trying new things. If I had your car to play with personally today, this is probably the first thing I would test. I would shut off the rears and double/triple the power to the front doors and dash. Set the eq FLAT, flat, flat, flat. Then I would bandbass the door speakers starting at 70-80hz with a steep slope, up to about 200-250hz. Start the dash ones from there up. Do this with the sub OFF, and take your time to get the levels and timing set between those four sets of speakers. This is where a dsp with a hand held controller in super snazzy, as you can dial from the listener position. Something like the Alpine H800! Anyways back to the suggestion. Once the fronts are dialed, feather in the sub. There should be very little need for the EQ, other than small adjustments. If they are more than a small adjustments something is going on elsewhere, such as reflections or harmonic distortion. Things that cannot be EQ'ed out, and MUST be dealt with accordingly. As always have fun with it!!
Anyways a few really good reads that apply perfectly to the info you are after!
http://www.glasswolf.net/papers/sqsystem.html
http://www.soundsolutionsaudio.com/forum/topic/72939-time-alignment-tutorial/