Best Way to Face Subs

You might try firing them down at the floor, leaving just a 2-3 inches of space between them and the floor.

This will accomplish two things:
1) Compound load them into the floor (compound loading is very much black magic.. doesn't shift frequency response, so let's stick with 2-3 inches)
2) direct the bass energy largely under your front seats - if you can further intentionally direct the energy that way it could prove beneficial - to give more of that "up front bass" illusion.

Really, firing them any other direction is not likely to actually gain you anything.

Firing them straight up, in fact, is likely to present the most direct path to your ears.
It's not a good thing, in that if there are any anomolies, resonances, harmonics, or other distortions of a higher-frequency-content nature (and most subs do), it'll ruin that "bass up front" image, because the human ear is progressively more locating sound source as the frequencies do increase (which is precisely why you set your LP filter on your amp around 80Hz or so)