Magnepan 20.7 - Ten Feet or More Into a Room?

In case this might help, it is well established that reflected sounds should have at least a 10 millisecond delay from direct sounds to help clarity and minimize sonic smearing. Since sound travels at close to one foot per millisecond that makes calculations and set up easier.

This is why it is good to have the side wall reflection distance at least 10' longer than the direct distance from speaker to listener. Also, with dipole speakers it is best to create at least a 10 ms delay for the back wave. That can be done easily by placing the speakers at least 5' out from the front wall. So that should be the minimum, unless diffraction devices are added.

In my experience with Maggies and other dipoles the size of the room best determines if greater than 5' may be beneficial. Unfortunately there are so many variations in actual dimension, room construction materials, furnishings, and listener preferences that further specific recommendations are difficult. I would guess that 10' to 12' could work quite well but the only real answer is for you to try it and see.

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Dude, post a photo of your room. Ten to twelve feet out, that must be a serious-sized room...I'd love to see a photo of it.
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My current room is 17.5w x 26L x 9h. I have the 20.7's angled in a bit with
the tweeter ribbons to the outside (yes, this gives a very slightly less
focused center image but it helps spread the sound field even further left
amd right; I like the 'big' soundfield). The inside edges of the 20's are 7'
from the front wall, the outside edges are 7'-9".

I am getting ready to build a new house and I have the opportunity to
design an even larger dedicated listening room. Initially it was going to be
19w x 29L x 11h. Further design changes and roof issues may allow me to
expand the room width by 2 additional feet, so width would be 21'.

So, at 3ft longer and 2.5ft wider I am thinking that I will be tempted to pull
the 20.7's another foot or three into the room. Since it is a purpose
designed, dedicated room I can put my listening chair anywhere (as long as
it works accoustically for good sound).

fwiw I thought my 1.6s sounded their best when using the rule of thirds in my 13 x 22 room, This means they were 7.3 feet from the back wall and 4.3 ft apart. The bass integrated with the midrange the best of any set up I have tried and the image was seamless all around and in between the speakers. I could not leave them that way because they were out so far but it sounded terrific! I ended up using the Cardas method which was a good compromise.

I hope this is of some use to you...

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Looks like that's going to be a killer dedicated room.
Congrats on your achievement.
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The answer is yes! Bass, however, might be less than satisfying. I've owned Magnepan products and can attest to the improvement that a quality fast/sealed sub can make.

Last year I added two REL 328 subs and they have really moved my rig to the next level. Bass from the 20's is very good but I was always missing that last little bit of energy from the mid-bass down. The mid-bass and down was tuneful, articulate, and 'tight', but it did not have what I like to call, 'grunt'. Drum wacks and bass guitar lines were there but they lacked that last level of realism.

Adding the REL's changed all that. Now when a heavy drum whack ocurrs you can feel the energy/impact hit you in the chest! Steady bass lines have that throbbing beat and energy that you hear in a good live show.

The REL's have also dramatically improved 'space' in most of the music I play. They make the soundfield BIG and all that space has become more dense with sound and detail.

Before adding the REL's I would often times switch over to my Khorns when playing a lot of rock music; now I no longer get the urge to do that.

fwiw you should try the planar asylum over at Audio Asylum

those guys are all over anything planar

When I did my room, my architect contacted an acoustic engineer who suggested room dimensions of 1.6 x ceiling height by 2.6 x ceiling height. It's a great sounding room. If you narrow your planned width by just a bit, you'll be right on top of those ratios.

You might also consider 2 more subs. Check out the Swarm subwoofer array from audio kinesis for the logic behind this approach.

Good luck with your project