Subwoofers - Front Firing vs. Downward Firing

Which is best? What are Pros and Cons of each?
Acoustically there is likely to be only a small difference in the bass. The downfiring sub will be close to the floor surface at a high pressure point - so you may get slightly more acoustic output (think how loading helps a horn) - although this will lower the F3 of the design and reduce the sensitivity (need a bigger amp) but the resposne will be flatter. The downfiring will also reduce the audibility of any small amounts of midrange that come through.

If you want to run the sub up higher in frequency then a front firing will have the advantage as you can place it close to your speaker and use it like a bass bin. (if you do that with a downfiring sub then you will tend to smear the midrange)
With a down firing sub you can also turn it upside down and place a board on top (with something heavy on it to hold it down). This will reduce floor bounce anomalies and tighten up the bass considerably. To do the same with a front firing sub you would need to raise it up off the floor on some kind of rigid platform.

Just a consideration.

Ptmconsulting: Your suggestion of turning the down-firing sub upside down interests me. Are you saying that this arrangement generally tightens up the bass some? With an active sub unit, are there any complications (e.g. heat dissipation, etc.) that one should consider before doing this? I’m interested in trying this with an REL Storm III. Thanks!
Bear in mind that by moving the woofer around you are altering the position of the bass driver. This is similar to moving a speaker or raising it on a stand. If your subwoofer is 2 feet high then if you turn it upside down you have rasied the woofer by two feet.

Two feet corresponds to a quarter wave cancellation of around 130 Hz ( a suckout ) - since most subs have plenty of output at 130 Hz then you will get the smoothest response by keeping it downfiring. If you tuck it up against a wall then you'll get the smoothest response.

Of course room modes will be more severe with a wall or corner placement - so "in theory" you can try to play around with position to get the most even response at the listening position by balancing quarter wave cancellation with room modes. In practice this is very hard to do as they hardly ever line up so that they cancel eachother over the sub bandwidth and you lose dynamic range in doing so (the sub is most powerful placed up against a side wall or corner). Generally a corner placement with down firing sub will give you the most dynamic range and smoothest response but this comes at the expense of overly strong room modes. Often a side wall placement with downfiring and pulled out from the wall where you have the speakers seems to be a suitable compromise - in this case the quarter wave cancellations from floor and side wall are eliminated and the cancellation from the wall behind the main speaker is not severe or deep because of the odd geometry between you and the sub.
As a general rule - as you move the sub out into the room you'll need to use a sharper low pass filter and lower cut off to achieve proper integration and don't forget to add delay based on the sub position to the listener => 1 foot closer = 1 msec of delay (roughly).