Magnepan 20.7 and Subs

Bdp-24 - When properly powered there is no speaker that does drums as well as a large maggie.  

@mcreyn, right you are. I use the Sheffield Drum Record direct-to-disk LP to evaluate loudspeakers, and that’s why I still have a pair of Tympani T-IVa. Nothing I’ve heard equals that speaker for the percussive attack of a stick on a drumhead, the hard felt beater of a bass drum pedal on a 24" head (I have recordings of my own Gretsch and Ludwig drums), and the hard felt of piano keys hitting the strings. It’s "pretty good" in other ways, too ;-) .

bdp24, I think you are confusing terms. The opposite of dipole is unipole.
Yes, Maggies are dipole throughout the frequency range as they radiate from both sides but 180 degrees out of phase. Bipoles are speakers that radiate from both sides but in phase. My sub design that we spoke about is a bipole. Linear arrays or line sources oppose point source speakers. To radiate as a line source a speaker must be taller than the wavelength it is to reproduce or terminate both ends at a boundary such as the floor and ceiling. My 2+2s are line sources down to 1 Hz because they do just that which is why I wish Magnepan had decided to make the 20.7 almost 8 feet tall. 20.7s are 6" 7" tall that would correspond to a wavelength of somewhere between 150 and 200 Hz. They are too short to abut an 8 foot ceiling thus they become point sources certainly below 150 Hz. Thus their bass power drops off rather dramatically at a distance. On top of this you have the issue with the front wall. If you put the speaker three feet from the wall sound bounces off the wall then rejoins the front wave 6 feet later exactly in phase with one frequency around 150 Hz but 0.6 microseconds late. Every foot equals 0.6 microseconds at sea level. At every other frequency it becomes progressively out of phase but still 0.6 late. So what you have here is smearing of the transient response and alternating amplification and attenuation of the signal resulting in rather monotonous bass. It is easy to dampen signals above 200-250 Hz but it is very hard to stop long wavelengths. Whether anyone likes this or not the best way to deal with this problem is to keep the dipoles as far away from the wall as possible and cross to subwoofers as high up as possible. Since Maggies become point source for sure by 150 Hz two subs in the corners will work fine. 
Now about digital bass management. You not only have to provide the right crossover for your speakers but you also have to make sure the subwoofers are in phase with the satellites. You simply can not do this in the analog world. You have to delay the signal of the satellites long enough so that the signal from the subs has a chance to catch up. The exact amount has to be measured with a microphone by impulse testing which the better room control units like the Trinnov and TACT do automatically. Some subs allow you to adjust the phase angle which is an improvement but it does not correct in the time domain which is ultimate. The Tact will actually allow you to use any seat in the room as the listening position by adjusting the arrival times of all the speakers so that they get to any single listening position at exactly the same time. Very spooky. So, you can have a preset for any position. If your friend wants to sit in the far left seat select the preset for that position and they will have a perfect image.