Magnepan 20.7 - Ten Feet or More Into a Room?

Polling Magnepan users with larger rooms; assuming that the overall room length allows it (and a dedicated listening room), do you think 20.7's can go as far as 10 to 12 feet off the front wall and still work well? At that distance from the front wall can they still fill the space behind the speaker and produce a convincing sound field?
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