My experience adding subwoofers to 2 channel

vinny I think you figured it out- when it sounds like just the mains, that's what you want. Some recordings have great low bass but a lot have very little or none. This makes it real easy to set the subs too high and not realize it until you get the rare record with really good bass. The meter test CD thing can get you close but then I think it just takes a while of listening and tweaking. Duke said the same thing, expect to spend a while fine tuning the levels. Then you put on Welcome to the Machine, Seal, Bird on a Wire, something like that, totally worth it.

    Yes, the downside of using a custom DBA with 4 amplified subs is the need to adjust the volume, xover frequency and phase on each individually and do a lot of tweaking til you get it just right.  

    Buying the complete Audio Kinesis Swarm or Debra DBA kit for $2,800 has the advantage of these controls being set on the sub amp that controls the configuration on all 4 passive (no internal amp) subs as a group.  You also only need a single a/c outlet for the amp as opposed to 4 outlets for subs with internal amps.
    My room has a crawl space below and I ran all the connecting sub and amp speaker cabling through that.  
   I also agree with setting the xover frequency as low as possible for best integration with the main speakers.

Tim

The plate amps in the Rythmik subs provide all the controls mentioned by @Noble100: volume, phase (180 degrees via a continuously-variable rotary knob), x/o frequency (40-120Hz) and slope (2nd/4th order), plus damping, a rumble filter, one band of PEQ, and both line level (RCA jacks) and speaker level (binding posts) hook-up. There is no reason four of them cannot be implemented as a swarm. You will however need four AC outlets. ;-)

It is not only phase but arrival times that are important. Two woofers can be in phase at a specific frequency but if one is say 15 feet from the listening position and another 10 feet from the listening position the arrival times will vary and the woofers will be out of phase at all other frequencies. This will muddy the bass and soften the impact of bass drums and such. You can fix this by using digital delays on the speakers that arrive first so that they all arrive at the same time but this works only at one place which is really all I care about. Everyplace else in the room is "background music." There is more to consider than just acoustics. The best approach is to make the woofers function acoustically as one driver which they will do as long as they are within 1/2 wavelength of the highest frequency you care to reproduce. So if you are crossing at 100 Hz the woofers should be no farther than 5 feet apart giving an array 15 feet wide. Below 100 Hz this array will function as one driver giving you one arrival time pretty much anywhere in the room. Now you just have to adjust the arrival time to match your satellites which you can easily do with a tape measure. Try it.

@mijostyn +1
a swarm is even bass, not accurate.