I've been advocating something like this for a while, since really there isn't much point to making a speaker go down to 20Hz since standing waves 'stand' a good chance of causing the bass to not be right at the listening chair.
A Distributed Bass Array (like the Swarm from audiokinesis.com) can take make the deep bass and break up standing waves. As long as the speaker can make it below about 60Hz or so and the DBA set up to not go above 80Hz, the main speakers will convince you that the bass notes are coming from in front of you and there won't be any bass problems, right down to 20Hz (at least in the case of the Swarm). The only issue I see that could be a problem is out-of-band bass messing with the woofer in the main speaker. But otherwise you should be able to put something together that would rival a speaker that costs in 5 or 6 figures unless your room was enormous.
A Distributed Bass Array (like the Swarm from audiokinesis.com) can take make the deep bass and break up standing waves. As long as the speaker can make it below about 60Hz or so and the DBA set up to not go above 80Hz, the main speakers will convince you that the bass notes are coming from in front of you and there won't be any bass problems, right down to 20Hz (at least in the case of the Swarm). The only issue I see that could be a problem is out-of-band bass messing with the woofer in the main speaker. But otherwise you should be able to put something together that would rival a speaker that costs in 5 or 6 figures unless your room was enormous.