Another +1 for Rhythmik (I have a pair of F12G's -- with the GR drivers). I use an external crossover to avoid overlap between mains and subs.
In general, I look at subs addressing two issues: First, and for some the only issue, they get the low end that your speakers are missing. Having an external crossover is a must.
The second issue is addressing room modes in a way that preserves your system's imaging. This can be complex to calculate, but there are techniques (look up the "subwoofer crawl" for example) to experimentally determine placement. And DSP systems may be able to help, although DSP is not some magic bullet that can make up for poor sub placement.
The beauty of using two or more subs is that they can cancel out areas of high bass amplitude and/or fill in areas of low bass amplitude. The sub placements that minimize room modes are highly unlikely to be the optimal placement for your mains. This is fine; as long as the crossover is less than about 100 Hz, most people's ears won't be able to localize the source of low frequency sound.
So with multiple subs, you can place the subs for best room mode mitigation, while leaving your speakers in the locations that provide the best imaging / soundstage. Even if you had magic 20-20kHz speakers, the locations of the speakers for imaging vs. the location for best bass won't be the same. You are better off decoupling that location by using subs and an active crossover.
Placed properly, two or more subs will make that soundstage seem much bigger; everything disappears and you're right there with the music. What has worked for me is two Rhythmik subs crossing over at 70 Hz to my Epicon 6's. Yes, the Epicons can play lower, but the room response means letting the Rhythmiks take the bottom end is the right thing.
I wish I could build a proper listening room, but that's not in the cards for me. Proper placement of multiple subs is a reasonable alternative to a dedicated room...
