Questions about 12 db and 24 slope

I recently upgraded to a new head unit in the car, and it's the first I've owned that offered 12 and 24 db slopes on both high and low pass filters (has to be one or the other if either filter is on)

All speakers are externally amplified between 3 amplifiers, 1 of which is for subs which I won't discuss. I have tweeters and two 8" woofers on one amp (Crossfire VR302) and six 6.5" on other amp (Crossfire VR402).

Before adding the 8" woofers (NOT subs), I ran both Crossfire amps with full-range settings and head unit set at 100hz high pass cut with 24db slope as four of the 6.5" are only rated to 100hz and above and they actually mean it.

The 8" woofers are rated 35hz to 3khz and are in separate ported boxes, and the 100hz cut is too high for them.

Crossfire states the HP/LP x-over uses a 12db slope.

Got enough back-story? :D

If I set the VR402's x-over to 100hz HP for the 6.5" family of 6, it's going to add a 12 db slope from the amp. And I'd probably have 50hz or 63hz as new HU high-pass so the 8" woofers cover more vocal and instrumental ranges, but I'm forced to run a 12 or 24db slope at the HU if I keep the high-pass on :headbash:

1. That's not going to work, right? Won't the HU's 50hz 12/24db slope interfere with the amp's 100hz 12db slope?

2. Is the only option to disable the HU high-pass and set both amplifier's high-pass cuts and just accept the 12db slopes both amps offers?

3. Also, what happens if HU and amp are both set at same freq. cut and BOTH are adding a 12db slope? Would it be cumulative?
 
Uh, what? Crossovers of the amp(s) and HU are passive, not active. 4 of the 6.5" are 3-way with built-in crossover caps but can not tolerate sub 100hz signals at higher volume.

I want the high-pass cut to drop from 100hz at the HU to 50-63hz at HU, so the 8" woofers can cover sub 100hz music (which is where the present cut is blocking them), but I need the 6.5" woofers to maintain a 100hz or possibly 125hz to cut to account for the 12db slope.

Was wanting to avoid turning off the HU's high-pass altogether, but that doesn't appear possible. Wanted to know what the effect of having 2 different sources with their own 12db slope would do.

Can you elaborate on how I'm all over the place? I'm running a true 2-channel stereo signal, so Left and Right is all I have to contend with. No fader, to extra junk coloring the signal, etc. 8" woofers are the only things w/o an actual choke and cap. They don't need it at this point.
 
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Setting HU at 50hz with the 12db slope doesn't seem to be interfering with amp and it's 100hz cut after a couple hour drive and experimenting.

It's just a daily driver and goal was to mimic a home stereo (vintage SS gear sound) as much as possible while staying coherent at louder volumes and avoiding anything in the line that would color or even dull the experience. With the way the speakers are mounted, it's fairly difficult to tell where the sound is coming from and I'm happy with the results.