My guess is that the problem is not the speakers, but the amp in the
headunit running out of juice and clipping.

However, as far as an x-over goes, you need to know that one capacitor
will only be a 6db/octave crossover, which means that with a 300hz
x-over point, it will only be down 6db, and then 150 will be down 12,
and 75 will be down 18.  I'd suggest something like a 2nd order, or
third order crossover.

A second order has a capacitor and inductor and has a 12db/octave
slope, and a third order has a 18db/octave slope (with three x-over
components).  Then a 4th order has a 24db/octave slope.

This is all immaterial though.  For a 400hz x-over point, you are
looking at 100uF (this is assuming a 4 ohm nominal load, which is
another thing to think about, the impediance of a speaker changes
depending on the frequency). For 300hz it's 133uF.  I'd suggest getting
a good audio quality capacitor from www.partsexpress.com .  The dayton
5% tolerance x-over caps are pretty good.

If you go with a more than first octave arrangement, you have to start
worrying about alignments, etc. Probably not worth it, but here is a
calculator to do it with http://www.lalena.com/audio/calculator/xover/
. Note too that a passive crossover introduces a phase shift,
especially with a second order.

Note though, that these may block the speaker, but your headunit will
still be producing the power to go to them, but it will be absorbed by
the capacitor and reproduced as heat (well, in a way, the impediance
will rise, so less current will be produced, but that is another
discussion), so that won't stop your headunit from distorting.

Hope that this helps.

what you need are non-polar capacitors. There is a formula for determining
the capacitance here -->  http://www.teamrocs.com/technical/pages/twoway.htm
Now this shows a two-way setup, but the tweeter section applies to you, only
you will be using bigger capacitors to get lower frequencies.

I had the exact same problem, went to an electronics store ready to
spend big bucks but they had cheap ($1.5) 68microF/100V NP capacitors
and got 12 cheap ones.  I used 3 per speaker (important: wire the
capacitors in PARALLEL to get 3X68).  The difficult part was mounting
them so they wouldn't rattle inside the door.  Use tape or whatever
else you want but be carefull because their cases are not very tough
and you can dent them easily (wires and pliers are NO).

I am now able to crank up the bass on the HU, the speakers don't sound
horrible (guitars have a plenty of body) and my sub in the trunk plays
all the bass.  If you use 2 caps per speaker you will find noticable
bass-reduction at the speaker and I didn't like that.  With caps so
cheap experiment.
--
Leon