Chops and Roberto have posted some very good info and reading. I want to add that if any of our ML high end guys are thinking of buying a sub, please get a high end sub. This means you can control almost all if not all of the subs features with a remote. Also, buy a sub a bit bigger than what you think you will need. The quality of your bass is MUCH better when using a bigger sub with less excursion than a smaller less powerful sub and really getting into more excursion on the cone. Currently many come with DSP built in and if your in that price range make sure you have it on what your looking at. Generally, and I hate to say generally but don't look at anything smaller than a 12 inch woofer sub. There are very good smaller ones out there but in general, a 12 or even better a 15 inch sub will give you fantastic performance. All in all, get a very good sub and enjoy!
Current cone/suspension/motor technology on subwoofer drivers these days, you don't always have to have large drivers to accomplish the same thing. There are plenty of solid 10" subwoofers out there that can easily compete with larger 12", 13" and 15" subs.
I'm not too sure in the home audio world, but in the car audio world, I know (and have heard) 6" and 8" subwoofers that are quite capable of large amounts of excursion, large amounts of power handling, and can dig deep into the low 20's, upper teens with ultra clean output at high volume levels, thanks to modern technology in their cone/suspension/motor systems. Some of them have clean Xmax up to or close to 2".
For a home audio example, I have a pair of JL Audio e110 subwoofers (the amps for both are arriving from JL tomorrow in fact) with custom built 10" drivers specifically designed for that model, and not only do they have 1200 watts RMS on tap, but the woofers are capable of 2.5" excursion. Short of going with servo controlled subwoofers, these little JL's are some of the most controlled, detailed and cleanest subs I have ever heard for home use. And with two of them along with room gain, they have no problems digging down well below 20 Hz. The driver alone has to weigh around 30 lbs.
If you're like me and your 2-channel system is used for 95% music, 5% movies, then a pair of very capable 10" subs will do pretty much anything you ask of them, including digging well below 20 Hz with usable output.
Now if you're into all out concert level SPL, and/or maximum slam and rumble with movies, then sure, multiple 12's, 13's or 15's are the way to go.