Ok..a real suggestion here man....It does, but I can’t hone in on limiting upper and lower frequencies. I do have passive crossovers in the doors on the front flutes. The rear ones are turned down, only for a little more rear fill and additional mid bass.
Switch over to network. Use the active xovers up front. For the back use a set of y adapters on the midbass rcas. That way you send the same signal to the front as the back.
Throw the passives or grab an inductor from parts express which is just a simple 6db low pass.
They would go inline on the positive wire. Like any other xover.
You could even order a few. Say from 200 hertz to 1000.
The argument for rear speakers is legit but...it's only 2 channel audio.
The rear info is the same as the front. In your case here anyway.
An actual ported box in the boot mate and you have a very nice tunable system that shouldn't be lacking in any real way.
The only improvements after that would be an actual dsp.
Maybe better speakers but only after the dsp has really maximized what you have.
Oh and one these