Is it possible that the spacious sound I get when my system is set up along the long axis is because the back wall, which is brick, is so far behind me (about 17 feet) and the reflected bass is not there to muddle things?
That was my first idea. It could also be that the brick is softer and less reflective than the plaster. If the plaster rests on a lath base it could have been absorbing a little of the low end as well when the speakers were pointed at it.
This could also have to do with your speakers now being nearer the open part of the room, emanating out into the stairflight etc and not being trapped under the trapezoid of roof.
As far as aesthetics, if there's other furniture causing the conflict, and some of that furniture is a pair of bookshelves, you could always try placing the speakers atop them provided there's no danger. That would ensure you have nothing between the speakers and is usually good for low end extension.