It's Magnepan 20.1's or B&W Nautilus 801's

I prefer the Maggie's for many reasons. However, your room could be problematic for the immense 20.1's. I think they need even more room to breathe and open up, but it might still be manageable.

You certainly have enough power to drive them, but I would also consider something a bit smaller. I am not a fan of the B&W's at all, so I will make no further comments regarding them.

You talk about a 10 foot peak in the ceiling, but what is the shortest part of it? Where you envision the speaker placement, do you really have enough height?

Your music preferences certainly lean toward the Maggie's. What did you like most about the B&W's? What did you not care for with them? I also would ask you to answer the same questions with regards to the 20.1's.

I agree with a lot of comments here but have to say I do have personal experience from the perspective of the same sized room (approx). I've owned the 801N's and Maggie 3.5's. I'd like to urge you to listen to these both before you purchase them IN YOUR ROOM. I know this may be difficult but I can tell you my experience with that size room, especially with the B&W's are that you will be encountering some standing wave/room reinforcement issues. 801's need a large room to breathe, maybe even more so than the Maggies. I had mine for about 6 months, tried everything to get the room to sound better, then sold them at a huge loss. The bass was so overwhelming that it smeared the midrange and the imaging.
IN retrospect I would have purchased the 802N, which I compared very closely side by side with the 801. It simply would have worked better in my room due to quicker, tighter bass. I ended up with Wilson Sophia's which I think compared very favorably to either : )
BTW, I miss the Maggies more than any other of the dozens of speakers I've owned over the past few years : ) As the reviews say, in some ways the best out there, even the 3.6's. I had them dialed in a "trick" room which opened to another room with part walls on outside of the speakers and managed a soundstage depth more than I've heard ANYWHERE (50 ft+) with NO room treatments. Rowland amps and pre, and accuphase and Goldmund digital. The only thing about the maggies is you need to get a good sub to integrate, and they don't do weight and body like good dynamic speakers. And slightly blunt leading edge transients. Everywhere else they rule though. Going to the 801's after the Maggies was a major disappointment. They sounded closed in/boxy? and lacked detail, transparency, soundstage, believability. Just my experience/opinion, may have been supporting equipment had something to do with also.

The Wilson Sophias are like a hybrid between the 801 and the Maggies. Very musical, imaging from hell, dynamics well beyond the 801, and sweet, forgiving sound like the 801. As others have posted, what counts is your opinion, your listening preferences, equipment and room. Either speaker would be terrific. It's a lot of money, so listen to everything you can in rooms as similar to yours with your equipment. I've trucked 200 pounds of amps into dealer showrooms so I could hear them on my own rig>

I was not able to demo the 20,1 but am making inferences from the MG 3.6R

Maggie Likes:
Sound Stage
Imaging
clarity of highs from the ribbon tweeter
very smooth midrange with good transistion to highs and base

Maggie Dislike:
apparently less dynamic range, perhaps a problem on sympnonic music which I don't listen to a great deal of.
Lack of bass punch and extension on some passages (hence the 20.1's instead of 3.6's)

B&W 801's

Likes:
Enough bass capacity for anyone and plenty of bass extension.No subwoffer needed here !
Dynamic range out the wahzoo - a sonic cannon
Excellent imaging for cone speakers but not as good as Maggie.
Clear midrange and highs and smooth transition between them.
Good soundstage - not as good as the Maggies, however.
Sounded better than Maggies on the demo pieces that emphasized the extremes in dynamics and bass, brass,

Dislikes:
Speaker gives less realism of "being there". Speakers did not "dissapear".
Much smaller soundstage
Very little else.

Concerning my 18 X 18 room with the "A" frame ceiling, I have an 18 ft flat wall and the ceiling connects at the 7 foot level and then rises to its cnter peak of 10 feet. This is where I would put the speakers. My listening position is near the opposite wall where the ceiling comes down to four feet at a 60 degree angle. Listenning chair is out about three feet from the wall giving 6 foot headroom and 15 feet from the opposite wall.

Roy3

Roy3- take a look at hiend dealer-Jtinn's system and you'll see he's now got a pair of Talon Khorus X speakers in his stable of mega buck equipment.

If you're not lock into Maggies or B&Ws, the Talon X or Ravens will give you all the things you like about the B&Ws plus a larger soundstage and would be a great match
to a BAT tube amp. Talon's build quality and customer support are also excellent.

Aloha.

Thank you all for your input and advice. I decided on the Magnepan 20.1's and placed the order today.

Best regards -- Roy3