Just to clerify though, your crossover slope will increase the cut-off rate. Isn't a 24db slope pretty much a dead stop at the desired frequency?The acoustics of many cars creates a frequency "suckout" right around 150hz. This is right in the middle of the frequency range you are having problems with most likely. This is caused by a combination of destructive interference from a standing wave node in the car and usually by phase cancellation between the sub(s) and the midrange speakers as they are both playing at this frequency. Remeber that your crossover is not a straight cutoff. The frequency that you select is where the halfpower point for that channel is (-3dB from flat). The sub is still playing above that freq but it is being attenuated more and more, the farther above the freq it gets. This sound can still play hell with you frequency response if it is badly out of phase with the mids. Try swapping the + and - wires on your subs and see if it helps. You can also experiment with switching the phasing on one of your mids to see if that helps. If you have a HU that does time alignment, that can help this problem a lot, too.