Magnepan 20.7 and Subs

I love 20.7s. Great speaker. They do bass better than most dipoles but still suffer a bit below 40 Hz. The cross overs are at about 450 and 3500 Hz and they are usually pretty slow. 12 db/oct. Putting subwoofers in will clean them up a bit and increase impact. You will need at least two subs and ideally 4. I would cross at 24 db/oct  100 Hz. I think the JL Audio f112s would be wonderful as a pair. But if you want the ultimate in powerful performances 4 all right up against the front wall with two in the corner and the other two equally spaced along the front wall. With the 20.7 driven by proper 200 watt or better mono amps you will be rocking for sure. 

Elizabeth, that bassiness you describe can be adjusted by moving the speakers away from the wall which in an attached condo might be a problem due to room. The bass that is annoying you is between 150 and 50 Hz. If you have a test record with frequency tracks you can hear exactly where the problem is. Now interestingly enough if you get two subwoofers and cross them just a little higher 125-150 Hz you will have more control of your bass just by turning the subwoofer volume down and get the advantage of further cleaning up an already clean loudspeaker. The longer bass excursions doppler distort the higher frequencies, it modulates them. It is one of the most important reasons for using subwoofers with planar dipoles.

Mike

don_c55, Magnepans are linear radiators down to about 150 Hz. Below that they are point source radiators just like any single sub woofer. Maggies will make bass but it is very lumpy for lack of a better term. Not only is their response not smooth but input below 50 Hz really shakes the speaker distorting everything else. I prefer 4 subs with 20 series Maggies using 12 inch drivers and crossing up at 125 Hz essentially keeping the system Line Source throughout. You also have to high pass the 20.7s to get the full benefit so you really need digital bass management to do it right along with room control. Now we are talking serious money. It is best to avoid subs unless you can do at least two. Single subs will just get lost and cause a lot of heart burn and frustration.
Setting up Maggies to get the smoothest bass is just a matter of adjusting the distance to the wall until you get what you want but it will never be the smoothest base because you can not avoid dipole interference and cancelation effects off the front wall. Wasting money on all kind of room treatments will not change or correct this. 

bdp24, I have owned my fair share of Maggies and have installed more than all of you have ever listened too. Yes they are dipoles and do not radiate to the sides but that has absolutely nothing to do with what happens at the front wall and how that affects the speakers bass response. I am not here to give you a lesson in acoustics if you want to learn more about dipole interaction with a wall there is loads written about it. You will never get completely smooth bass response out of any dipole unless you have it out in open air or live in an alternative universe. 
Having said all this there are very few loudspeakers I would choose over a 20.7. I just wish they had decided to do an 8 foot version. That would have been the nuts.

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. 

No they are not and I agree. I have never heard a full range bipole speaker that I have liked. Although I think unipole line sources are a step in the right direction all the ones I have heard have suffered next to dipole ribbons and ESLs. Variable phase in a sub is also a step in the right direction but very difficult to set up by ear and it does not correct the time issue. Assuming the Subs are behind the satellites as is usual. Even though the phasing can be correct the timing can not be. Using digital delay on the satellites works perfectly especially when you have a computer making the adjustment. We use to move the subs back and forth to get them in phase but as soon as you separate them from the wall you lose power, a no win situation. As for keeping dipoles away from the front wall it depends on where you want to amplify the bass. 5 feet will give you a peak at around 100 Hz which will give bass more impact but below that things will roll off quickly. You also interfere with the room more. Again the best solution is to move to a subwoofer array at 125 hz which gives you much flatter bass and allows you to keep the dipoles closer to the wall. I always dampen the wall directly behind the dipole with acoustic foam tiles which kills the primary reflection above 250 Hz tightening up the image. There is no significant radiation in any other direction except straight forward. Perfect!

Wharfdale did the sand. Bozak's claim to fame was infinite baffle speakers. There are lots of sub kits out there now and if you have some woodworking experience and some basic tools by all means. Dayton makes a very advanced plate amp with room control.

It is extremely difficult to control resonance in box speakers. The kit manufacturers because of weight limitations and cost to not dig very far into the problem. If you are capable,  making your own enclosure is the way to go. Calculating volume is easy once you know your driver's parameters. It is just plug in math. Tricks to control resonance are first use very thick walls. In MDF 2 inches minimum. Keep the sides, top and bottom as narrow as possible but make the top and bottom wider than the sides are tall so that they resonate a different frequencies. You want to make things so stiff that the cabinet resonates at a frequency as far above the woofers cut off point as possible. The heavier the better. You do not want the whole cabinet shaking. Always three spiked feet and right into the floor. No pads. Ideally with the woofer pumping out it's lowest frequency you should feel nothing with your hand on the enclosure. Any movement or vibration that you feel is distortion. You will never feel a commercially made sub that quiet with maybe the exception of the big Magico subs. Get a piece of granite cut to size and silicone it to the top of your enclosure. The added weight always helps and you can get granite cut offs for cheap. I prefer external amps and cross overs. JL Audio makes a nice external crossover.