So I currently have a 12" CompQ with signal coming from LC2i tapped into the stock rear 8" subwoofer.

I feel I lose a lot of volume at the higher end of low frequencies.

Does anyone know what frequency range is being sent to the 8" stock subwoofer? Trying to deduce whether it's an enclosure issue or whether the signals going to the amp aren't 'full-range' low-end frequencies.

PS - the front door 6x9s, anyone know these ranges too?

 

What amplifier are you using? My guess is the sub output is filtered above about 120hz no low end filtering. Is the stock sub still active? Try disabling it.

 

JD1000/1. LPF set to 80hz. But high-end of low, I still mean well below the 80hz.

My big question is regarding the output of the 8". I know the 6x9 provide midbass, but I want to see if there is a mismatch in my expectations.

 

Try hooking the factory sub to your amplifier see what it sounds like. Is the subwoofer broken in? This takes awhile won't sound great out of the box, same for the amp. You won't want to hear it but a class D amp won't sound nearly as good as class A/B even on the sub channel, I've tested this going class A/B GREATLY improved my bass. I'm using a 4-channel bridged into two 8" subs, Nakamichi's out of an LS400. Not very deep extended bass but otherwise they sound spectacular.

100hz is what I generally set the pass filter to on an 8" or 10" sub.