Magnepan 20.7 and Subs

I've been reading about supplementing my Magnepan 20.7s with a sub, the general preference being REL.  However in room I'm getting fairly deep bass with the 20.7's, so my sense is that I would put the crossover on the REL pretty low, probably at 30Hz, leaving only sub 30Hz material porting "also" to the sub to augment the natural deep bass of the Magnepans.  I'm just wondering since there's precious little info in the 15hz to 25hz range, do I really need a PAIR of RELs, or could I get by with just one?  And shouldn't that one be capable of going REALLY low, like the G1, or what's the point?  Any advice would be appreciated.  Thanks.
any new developments on this thread? Hope you are well and enjoying Summer.  Give us an update as your Maggies continue to bloom and settle into your room/system.
Happy Listening!
The 20.7 will go down to 25Hz in a good room with proper setup and a decent power amp.

There is a noticeable sonic transition to subs with tall Magnepan’s, because Magnepan’s have tall bass waves, and subs have short bass waves.

Some do not like planar bass, with it’s natural sound for acoustic instruments, they like punchy, dry bass, that box speakers put out with with rock ((since rock bass is played mostly through boxes.

I have Magnepan IIIas and 3.6s and guests always ask me where the sub woofers are?
There are none! I know how to setup Magnepan’s in an appropriate room.
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. 

Don, I share your amusement at you being lectured to on the subject of Magneplanars. Those who know Don are aware of the fact that he has forgotten more about that loudspeaker (having owned them since the 1970's) than most will ever know. Don also has a ee education, and it is HE who should be giving the lecture.

Don could talk about the fact that below 150Hz the Magneplanar bass panels (separate in my Tympani T-IVa) continue acting as dipole radiators, with a null to either side, therefore exciting one less room dimension mode. There is one hi-fi enthusiast (I need to find his name) who uses Martin Logan ESL's as his mains, Tympani bass panels as woofers (it reproduces the sound of my self-recorded Gretsch 26" bass drum more like it does in life than any other reproducer I've heard), and an Eminent Technology TRW-17 Thigpen Rotary Woofer as a "true" subwoofer (20Hz-down!). Now THAT is a system I would love to hear.

There is no need for "digital bass management" to high pass the 20.7; a First Watt B4---all discrete, no opamps, no ic's---works perfectly. So will a Pass or Marchand x/o. If a 1st-order filter is steep enough for your needs, you can even install a simple capacitor/resistor filter on the input jacks of the amp powering the 20.7's.

This is all old news to Maggie enthusiasts, and much more info about the speakers is available on numerous forums, including the Planar Speaker Asylum.

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.