nondirectional below 80hz?

I have, actually. The sub/PR is 'tuned' to about 40Hz, anything above that doesn't sound too great. I mean, it's there and all, but when I do that, it's like it is trying to reproduce too much, so it doesn't go as low as I like. That's why I'd like to add a dedicated 40-80Hz sub.

OK I'm coffused here. The port is at 40 Hz and you say it dosen't sound good above that? Bolow the tuning frequincy the speaker is basicaly at zero resistance box wise and running as if it were infanate baffle...please correct me if I'm wrong on my therory here. Does your woofer have a lot of movement when you are playing it? Dose the speaker move less when playing matterial above 40 Hz?

Always will to learn something new...

Its a woofer and PR (passive radiator) setup, sealed box. The PR is tuned to about 30Hz.

The PR moves more at lower freqs', the woofer at higher ones.

I think my car peaks at about 60Hz, actually. It is pretty bad. It's hard for me to get a flat response down to 20Hz. '95 Cavalier 2dr.

https://i209.photobucket.com/albums/bb103/jmelton86/subbox.jpg

That's a sheet of 1/2" plexi. It keeps the trunk lid from sounding like crap. I actually think it helped with low-end performance.

in what way does it not like the higher bass.

As you move closer the th P/R tuning frequency the P/R will do more work and the powered woffer will do less. But below the tuning frequency say two octives it become unstable. 30 Hz is one octive down now so I'm curious which woofer is doing the work?

80Hz is not "high bass."  80Hz is a subwoofer frequency and setting your LP for the sub and HP for your mains at 80Hz is the preferred setup for most applications.  Depending on the woofer you have in a PR alignment it should "like" anything from 400Hz or so down.   I humbly suggest you get your vehicle to a professional shop and pay to have it metered and to have your crossovers properly set for you as it's obvious we are not helping you here.

Something is deffinately not adding up here. DYohn is right the crossover setting 80/80 or even 100/100 are  very typical and should be working for you. So it is possibly time for a second set of eyes to look things over.