True 20Hz bass is going to cost a lot of money, either with large, expensive main speakers or a very good subwoofer. Since you seem to have movies in mind a subwoofer might work out. If you want true deep bass with music, well, that's another story.
Personally, I don't like subwoofers. Unless you get a really, really good subwoofer, with a cabinet built like a bank vault, they never seem to blend in seemlessly with the satellite speakers. A sub should never appear to have any sound coming from it. Worse than the localization problem is the level problem. It's just too tempting to turn up the bass too much, which is great for special effects, but sounds terrible in the long run on acoustic music. Getting the level right often takes an analyzer, which means more money. Some dealers will set things up right, if you spend enough money with them.
Of course, to add insult to injury, there's the damn crossover point and the crossover itself. An 80Hz crossover point means you've messed up the lower range of a grand piano (because an 80Hz crossover will have effects in the 80Hz-160Hz range). I'm a piano fan so this won't fly with me. So you make it an octave lower (40Hz), but then you've spent big money on a big sub you'll only hear contributions from in action movies now and then, or perhaps on organ music. And, to properly use a 40Hz crossover point you need main speakers that are nearly flat to 40Hz, which means you'll have big, expensive main speakers on your hands. And most of the time the system will sound better without the sub and it's crossover in the signal path!
My advice is to raise your frequency response target a bit, from 20Hz to 32Hz. There are many of good floorstanding speakers on the market that'll reproduce a pretty good 32Hz signal. 32Hz, properly reproduced with low harmonic distortion, shakes the room and makes your ears feel funny. 32Hz is deep. So if you take the money you would have spent on a sub+mains and spend them on just mains you can probably afford 32Hz mains, and the midrange and highs will probably be reproduced better too.
You didn't mention your budget limitations, but if you can afford them I'd strongly recommend you take a look at the Legacy Audio Classics. A friend has a pair as the mains in his home theater system, and they reproduce a very clean 32Hz signal that shakes the room. And they sound simply awesome on acoustic music too. I'm not sure what the price is any more, but I'd guess $2500/pair or so. I see mint used pairs on Audiogon every so often, and a different friend just picked up a 2yr old pair for about $1500. Even better, they're at least 3-5db more efficient than most other speakers, which has the same effect as doubling your amplifier power, or more. (Be forewarned, though, very efficient speakers reveal more noise from your electronics. Adding 3db of efficiency to your speakers has the effect of worsening the s/n ratio of your electronics by 3db (which DOUBLES the noise level).
www.legacyaudio.com
Of course, if you're really looking to spend less than, say, $1000, I think you're going to be disappointed. Cheap speakers that go deep are generally crap. Good low bass means you need at least a 100db capability at 40Hz at one meter, with low distortion, and that costs bucks.
