I don't know if that title is a good description, but I'm running an 880prs head unit with 3 way active crossovers built in. I'm planning a 4 way system (speakers should be in the mail by Monday/Tuesday) for my '11 Dodge Challenger. I'm blowing a bunch of money going for a dream SQ system. It's an esotec 362 with the 8" midbass, 3" midrange, and 1" tweeters. These will be run off a Zed Leviathan III. The substage consists of a pair of infinity perfect 10.1's in an IB/AP enclosure in the trunk firing into the rear seats. I say IB/AP because I've got something like a box/amp rack almost finished for them. You guys tell me what it is when you see the pictures below. Those are going to be getting power from a Hifonics 1610D run at 8 ohms (or 2 ohms with the gains set low).

My question revolves around the passbands and how to set this up fully active. I've got the amplification to do it. I've got some issues with the 880prs though in what I can do with filters. The 880's "low" channels have a maximum LPF of 250 Hz. The "mid" channels have a bandpass ranging from 25-12500 Hz (as narrow as 250-1250 Hz). The "high" channels can highpass from 1250-12500 Hz. I like having the adjustment built into the deck for quick adjustments on the road (at lights), but the head unit has seen it's better days (I've fixed 3 things internally on it, with a faceplate motor that's making noise now). I may be open to replacing it down the line, but for now it's free, and nobody is offering much in the way of good in dash audio processing that I'm aware of. I've got a double DIN opening now, so if anyone knows a good SQ deck packed full of features let me know!

The Hifonics has a full range passthrough for the signal. The Zed does not. The Zed has been customized to allow a bigger filter range on the low channels (knowing I'd be running 3 way frontstage later). Any way you slice it, I'll need to cross the signals of 2 channel pairs with the amps (subs/midbasses, midbasses/midranges, or midranges/tweeters I assume) and won't have quick access to changing them without getting into the trunk.

Let me sum that up again briefly for quick reference and add in filter bands for the amps:
"low" - LPF 25-250
"mid" - HPF 25-250, LPF 1250-12500
"high" - HPF 1250-12500

"low" - HPF (subsonic) 10-105, LPF 40-1000
"mid" - HPF 55-4000, LPF 55-4000
"high" - HPF 55-4000

Hifonics: LPF 55-250, HPF 15-35 (subsonic; with full range RCA passthrough)

MW172 FR: 35-3500
MD142 FR: 700-6000
MD102 FR: 2200-30000
Infinity Perfect 10.1s fill in the bottom end.

So, what's the most efficient way to cross this system? Are Y splitters a bad idea for an SQ system?

Pics of the amp rack/sub "enclosure" (under construction):
The trunk view:

I still need to make trim for the rectangles, speaker cloth the left and right rectangles, and possibly go more AP with a resistance wall inside. It's leaning more IB/bass tube like this right now.

The goal of this trunk install was to keep enough room for my toolbox, 2 sets of golf clubs, backpack, and easy access to the spare. I'm not looking to compete, but I'm shooting for a competition level system. The substage equipment would have to change to get it up to competition level, and the install would need to be cleaned up quite a bit. For me, it looks pretty and should be very functional.

Better shot of what's going on inside the box:

The amps are enclosed on the horizontal walls, but there won't be any more to the top. The subs vent upward out the box top and out the rectangles from the first picture. The trunk lighting casts light down on the amps just right. I may change the bulb for effect.

Overhead shot of the amp rack area:

The hifonics isn't mounted yet, but it will go in that empty space there. The amp rack is on hidden hinges so I can pull it forward and make adjustments to filters, phase, etc. It's a snug fit as is thanks to all the padding and the suede used, but I may add a rotating lock if it proves unstable without it. Both of these are highly efficient class D. I'm hoping heat won't be a problem with the open top, but if it is, I can add case fans.

The front baffle board:

A beauty panel cut and wrapped:

I don't have any shots of it installed because I'm not there yet. Hopefully I'll have a substage playing by the end of the weekend or end of next week.

I'm thinking the best channel pairs to leave to the amps will be the subs to the midbasses, but time alignment gets sketchy. The midbasses are going in the factory door locations (about ankle level). The mids and tweeters will share a vertical plane just below eye level in the A pillars, so their TA should be nearly identical, but obviously this is one where TA is important. I'm not married to the A pillars yet, but it's my #1 candidate. There's not a whole lot of info out there about the challenger and what works best so I'll have to put in some listening time before picking mounting points. I'm scrapping together a small amount of deadener and CCF I have around from my last vehicle's leftovers. It'll probably need more of both, but I don't want to add 20 lbs to my doors again. I'll probably seal the midbasses and spend my deadener on the doorskins and sealing up the sub baffle.

Any opinions on setting up the crossover functionality, or general guidance? Particularly about the sub "enclosure". It started strictly IB, but my amps are just slightly too big to put anywhere else without taking up too much trunk space or ditching the spare.