Any Car Audio Gurus Out There?

[BrianG]

Yeah, and the Ndb/oct is the slope of the cutoff.

12db/oct in a HPF means that for every halving of the frequency, the output will drop by 12db.

12db/oct in a LPF means that for every doubling of the frequency, the output will drop by 12db.


So, even though you have a HPF set up, the speakers are still getting some low frequencies and could be overworked trying to produce bass frequencies. A steeper slope (like 24db/oct) helps. 6db/oct (which is what you get using a capacitor on a speaker) is unsuitable IMO - the slope is not very steep.