(Edited for typos)
My advice, and I expect you'll get the same from others, is you have clearly made some improvements, now pause for a moment, spend time listening to your system, read about driver selection, and study, study, study installation and tuning methods before you pour any more money into components that may not work well together.
A few starting concepts, and you can research further from here:
Low bass frequencies, especially when played in a small room, are omnidirectional and cannot be located by your ears. Don't worry about subwoofer placement so much as installation technique.
Midrange and high frequencies contain lots of imaging cues and are easily located by your ears, this is in part a function of having shorter wavelengths relative to the spacing between your ears. In other words, your brain can interpret the intensity difference and the time delay between when the signal reaches your two ears, and instinctively direct your attention to the origin of the sound. For this reason, placement of these speakers is key to recreating sound that is similar to what was recorded.
It's worth trying the following: wrap the backs of your midrange drivers in a few layers of towels to cancel the back wave. Cut holes in tennis balls and install your tweeters into those holes. Wire both up with extra length of wire, and move them around the cabin, placing them close to any feasible locations where your could install them, and listen for differences. You can temporarily prop these up with twine, tape, Velcro, putty - whatever makes sense and will hold each speaker in place long enough for you to listen to a familiar recording and form an impression of how realistic or unrealistic the sound reproduction is. Repeat this exercise until you've tried every feasible location and angle for each driver, and identified the "sweet spot" where everything sounds nearly as good as you hope - with no signal processing whatsoever besides basic crossover setup either via passive crossovers (if your available number of channels of DSP and amplification are limited), or with the active crossovers built into your DSP. Proceed to install in those locations.
Setting crossover points is another whole topic. Your best starting point is to look up the Fs (resonant frequency) of your tweeter, and set your crossover point to 2x Fs. On the flip side of that, you'll want to consider the size of the drivers reproducing your midrange and woofer or "mid bass" as we sometimes call it, and consider how high each of these will be required to play based on the tweeter crossover settings.
You can calculate (searchable, though I don't store the formulas in my active memory) based on the size of the midrange and mid bass drivers where you can expect to hear cone breakup, beaming, and cancellation - effectively wherever frequencies from one side of the driver will cancel out the frequencies from the other side based on the distance between them adding up to half the wavelength of a given note. Manufacturer's frequency response charts, if available, will also shed light on the range in which you can expect a flat response - especially if a chart is provided for both on-axis and off-axis response. Try to cross your mids below the upper end of that flat range depending on the orientation. (Unless you get fancy, mids and woofers will almost always be mounted off-axis in the mobile audio environment, i.e., a typical door installation.)
If you have a 2-way setup and find you have a frequency gap between how high your woofers play and how low your tweeters play, then you have entered territory where a 3-way soundstage is appropriate and perhaps necessary for satisfactory results. You'll have to make a studied decision on whether or not that is appropriate in your build.
Once you understand everything I typed above, as well as all follow-up questions that have arisen, THEN commit to your next set of experiments. In the meantime, continue listening to what you currently have installed to establish a baseline you can refer back to, and detect whether any changes you make going forward lead you toward or away from accurate sound reproduction.
$.02.... my first stereo had two Auto Zone "bass tubes" running off the back two channels of an 80 watt head unit, with a pair of door mounted no-name coaxials with Mylar tweeters in ugly wooden spacers in the front door cards. We all start somewhere, and we all go through many iterations to get to something satisfying to our ears. The trick is, as we learn more, we hear more. It is an endless rabbit hole, but if you're going to head down it, you'll do best if your next attempts are studied and intentional, so you can understand why you get the results you do.
Good luck! As you can see, those of us who have been around this block a time or two are all very willing to help out where we can when someone presents a genuine question. We blend art and science here to a large degree, which means you'll often find more than one approach that works. The trick is gaining the background to understand why something does or doesn't work, and the craftsmanship to present it in a way you can personally enjoy and be proud of. Hope this and the comments you've received from others have been helpful.
-TJMA
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