Any Car Audio Gurus Out There?

[BrianG]

That will definitely help the main speakers from trying to reproduce any bass, but don't go too high because if there is too much of a gap between the main speakers and the sub, you'll notice a gap in midbass sound. And if you increase the LPF filter for the sub, the higher frequencies produced by the sub will pull the stereo imaging towards the sub (mids/highs are directional while bass frequencies are more omni-directional).

I would go around 140Hz on the main speakers and around 120Hz on the sub, even at 18db/oct.

EDIT: I just re-read your last post. Just because the HPF tops off at 180Hz doesn't necessarily mean it's 18db/oct. You should be able to find what the actual slope is for your device via a google search or the device's docs. Common values for "active" crossover slopes are 12db/oct (so-so), 18db/oct (good), and 24db/oct (best). Slopes higher than that are good too, but aren't common. "Active" crossovers are ones that use DSP or other active (powered) circuits to handle the crossover, and are done before the signal gets to the amp. "Passive" crossovers just use capacitors and inductors (and sometimes resistors too) between the amp and speakers, and do introduce insertion losses and can also reverse the phase, so are more difficult to build correctly.