Quality crossover and car audio crossover is an oxymoron. A single cap can be a crossover, but a simple one with a 6dB slope. The manual didn't post right, but I don't need one. Designing crossovers is not rocket science, but a good one is usually as expensive if not moreso than the drivers you use.
Any and EVERY driver should have a crossover on it. Otherwise it is going to be playing all frequencies. 20Hz in a tweeter is worse than 5kHz in a sub, but your response will be a hot mess if you don't use a crossover to optimize what speaker plays what.
A quality passive and a well setup active will mimic each other perfectly. They are electronically analogous. The only problem is you do not have good passives so your question is moot. EQ is also not for compensating for location or reflection. It is mean to allow you to cut out anomalies in the system. Those could be caused by either of the above, but it is a last resort not a first step.
Flat? NEVER. Why do you care what the response of your stereo is if YOU find it perfect. You should also generally NOT have to adjust for source material. That shows that the calibration of your system is way off. Do note that of course you will hear differences in sound, but it shouldn't be as drastic as you state.
One 4x10 sucks donkey. In fact there is no such thing as a good 4x10. I have to apologize in your first thread I really, really, really thought you were a troll posting misinformation and confusion to get a reaction. I now see that it is regrettably real.
To answer this last question, I want to start with a question to make sure I go back far enough. Do you understand constructive and deconstructive interference? If not, google it and then come back with questions regarding it and it will make it very easy to clear up why multiple drivers are not a good thing. You have an amazing amount of work in your car and have done a really nice job installing things; however, the information that led you to think you were doing something that would help your sound is really far off from reality. Somehow along the way someone really confused you and caused you to spend a superfluous amount of money to have very lackluster results. We can fix it, but first you need to understand why.