Magnepan 20.7 - Ten Feet or More Into a Room?

.
Looks like that's going to be a killer dedicated room.
Congrats on your achievement.
.

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