In my experience, the only way to get away with a mono sub is if the crossover point is exceedingly low, meaning less than 40Hz. Anything above that and you will be able to localize it. I also agree about the virtues of downloading; it seems to couple to the room better, but again will only work well with very low crossover points.
One woofer or a pair?
Theoretically speaking the human ear can only distinguish a stereo sound from mid to high freequencies.
What's the benefit of placin a pair of woofers for a pair of speakers?
What's the benefit of placin a pair of woofers for a pair of speakers?