Magnepan 20.7 - Ten Feet or More Into a Room?

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.