Without getting into a lengthy discussion about how to properly build a box determined by calculations using the Theile Small Parameters it just depends.  The parameters allow for proper fitting of woofers to available air space aka size of box in cubic feet.  It will also determine how hard or low they will hit. Box design and loading as you mentioned firing down also determine "impact".  There are sealed, ported, 4th-8th order "bandpass" boxes, horn, slot loaded, labyrinth which aren't usually applicable in auto situations and finally you can isobaric load any of those.  Isobaric is where you load two woofers face to face or one behind the other and it allows for smaller boxes and harder hitting.  So my box is a straight forward sealed box but we are building a 4th order bandpass with a single Orion 10" 1000w in it for the boy.  Boxes also force how well the bass is especially bandpass stle.  For example my sealed setup although it has a better QTC (quality of bass response across the bass frequencies) is 3 dbs down 42 and the single or 4th order bandpass (sealed on one side and ported on other) box for the boys Orion 10" will come out to be .5 ported chamber and .8 sealed chamber and will be 3 dbs down at 28 hz with a higher spl range than both my woofers between 30hz - 60hz.  His will be horn loaded off the rear hatch too increasing the time to travel before hitting occupants.  Basically the wavelength of bass is non directional and for the lower frequencies takes more time to develop and why often it's heard further from the vehicle.

Sorry to blab.  So it depends on the type of woofer, box and even more so the parameters of the sub woofer.  The type of music u listen to also has impact on what box to make etc..... Not all subs are created equal or have good parameters for every situation.  Certain woofers are better sealed than ported, vis versa etc.....  I can run whatever woofer you have and tell you the best box you could use for anyone who wants me to.  I'll just need to know the model number of the sub and what your listening habits are aka what type of music do you listen to.

BTW, back in the late 80's before bass offs were norm I had a car with 16 12" woofers isobaric loaded with 5000 watts behind it producing 149 dbs.  

Anywho I ramble, love the topic!  Let me know if you want me to run a sub in a certain box!