Magnepans in 18 x 35 room

Four-six feet out...six-eight feet apart, I'd start at five feet out. You are trying to do two things here: Adjust for best bass response, adjust arrival time of the rear sound wave in relation to the front (direct) soundwaves arrival time.

Spread...start at eight feet apart (no toe)...play music with good vocal reproduction, small jazz bands, ect. Adjust spacing if needed untill center image pops into place.

Tweak by adjusting toe-in if needed...the amount of toe-in needed (if any) will be determined by your seating distance.

Good luck...take your time, work on one part at a time.. and have fun.

Dave

"I have started the process as suggested with the rear of the speaker facing the 18' wall. Even though the space is so large, the seating area is at one end (where the speakers are and is roughly 18 x 15. Would there be any advantage to placing the speakers on the long wall, but at one end (within the first 15 feet)?"

Advantage, maybe...I would try it if it results in better seating. Because one side wall will now be live, and the other "not there" so to speak...I would try to stay as far away from the live wall as I could...some treatments on that wall may be needed for balance...your ear will answer that.

Because your room is wider than most...long wall placement is worth checking out...18' is still a good amount to work with. My speakers are on the long wall in my room...a fairly large room.

"Depending upon where you are on the L-shaped sectional, there can be significant dominance of one speaker or the other"

Maggies have a smaller sweet-spot than my speakers when I've listened to them. Better seating arrangement may help some, toe-in will help some, speaker spread could help some...increasing your listening distance would help a lot. Take your time and you will be rewarded.

"Lastly, I am not sure what effect the addition of a subwoofer might have in this space. Any advice would be appreciated."

Well, I have three of them. They are in my hometheater at the moment but I had two of them hooked into my two channel system (again) not long ago. I've had good luck with them in my system as long as I keep the X-over set pretty low, depending on which Maggies you own, where you can place the sub, and how you set the crossover I would expect you would have good luck in that room also.

Dave