Room help

I'm not sure we can replicate this exactly, but start with diffusor panels behind you and between the speakers.

My feeling is you can replicate better sound in your earlier orientation. Here is what I think… you can see my room under my ID. Consider putting up some photos.

First… if possible put the brick in front (to quiet the sound stage)… but also cover with a sound absorber (I use a heavy wool tightly knit carpet)… this will improve depth of image as well as widen.. You want to highly dampen the wall behind you. I suspect this is where you are really losing in terms of this location. Only an amp in between speakers. Heavy carpet between speakers and you.

That is the “big” stuff. Then you really want to fine tune… a diffusion tube behind each speaker and center behind amp. Tube traps in corner. Absorption bolsters along wall floor corner in front of listening wall. Finally corner diffusers at all wall - ceiling corners.

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.

Forget WAF. Orientation with the speakers facing the long direction and space behind you is correct.

Diffusers are a band-aid.

Still struggling with a similar situation myself. Suffice to say it’s complicated. Our ears interpret reflected noise and try to line them up. At best, we interpret room reflections in such away that the sound stage is wide and - most importantly - deep. I believe any treatments or dealings with the brick wall would have an absorbent affect and would not give you what you need - which is the even order harmonics that trick your brain into thinking the sound is coming from behind the speakers in front of you. You could get a cheap mic, test your current set up, reconfigure, test again, and then keep moving around until your long wall configuration matches the results of your preferred sound. My gut is that you will need to pull your seat out further from the back wall, push your speakers further apart and into the corners, and still you will end up with a dead space between the speakers where the bass should be. 
 

Absorption will only get you so far here. It will remove unwanted reflections. What I think you need is more correct reflections that the width of the room doesn’t allow. To test, try moving two large surfaces into the room - bookshelves or plywood. From your seating position, look behind you and imagine a 2 bumper pool shot that would get you back to your speaker - 1st hit the back wall then hit the temporary panel. Make sure the panels are out wide enough not to be inside the 1st reflection point zone.
 

This is what your narrow setup is doing. The sound hits the side walls, then the back wall, then your ears. You interpret the sound as coming from the speakers in front of you because it hit 2 surfaces (1 surface would cancel the sound out) and since the sound took so long to hit your ears, you interpret it as coming from further away - ie deep behind the speakers. Because bass frequencies hold up better in time, the bass tends to step further back in the sound stage - and now the instruments sound more separated. 
 

I have gone so far down this road as to add rear surrounds out of phase with the front speakers to aid in the illusion. The are directed at the space between the speakers in front of me. This actually works but at the cost of imaging. 
 

I’m not an expert by any means - but I would say try to experiment and do what you can to understand what is happening before you start purchasing permanent room treatments.