Speakers with no rear wall, better or worse sound?

You want the speakers you mention at least 5 feet out from the wall. The more the better. Mine are 7.5 feet and the sound is glorious. I measure from the front of the speaker. Perhaps 13 is too much, but I would have to hear them. The rule of 1/3's works very well based on my experience and current set up. My room is 24 feet long and my speakers are 7.5 feet out from the wall.

I bet 8-10 feet would be ideal for you. You should still have body without bass overhang and great imaging a focus.

Putting up a wall behind your speakers will lessen the perceived depth. Whether it's a visual or acoustical thing I don't know, but you will notice an amazing reduction and probably not like it at all.

Best soundstage depth I heard out of my system, I placed my speakers a third of the way into my 13x18 room (on the short wall) with a wide, open double doorway behind them, a 10x13 entryway beyond that, and beyond that through another open set of double doors, the living room. The distance from the speakers to the nearest full rear wall was 36 feet. Despite the speakers being closer to side walls, I do think I was hearing the depth of the soundstage on each recording more accurately than I ever had. Unfortunately, bass was MIA (lacking body and depth) and the placement was a bit awkward to live with.

I'd imagine you're probably getting most of the benefit already with a 14 foot distance? You could try some diffusion on the back wall--like a jungle's worth of plants. They help a lot.

Ablang

I really can identify with your situation which sounds quite similar to my set up. We also have a double door opening (French doors removed) 3 feet behind the speakers with about a 3 foot wall section flanking the double doorway opening on either side. There is another room behind the speakers with the back wall mostly glass. The depth of the soundstage is way back into the second room and bass is not all that bad to my ears. I have been speculating that the combination of a partial wall with the large double door opening and lengthy distance to the rear wall might be an 'ideal' mix of minimizing reflected sound behind the speakers without losing too much 'body' or room ambiance.

Hickory, I just wanted to add my voice to those who say that you'll get better depth if you have your speakers away from the wall. It will increase until they're about 15' out. The same thing is true of width and side walls. The reason for this is that we judge the size of an acoustical space by the time delay between the direct sound and its reflections. Bigger room, longer delay. Since most recordings are made in fairly big spaces (or have artificial reverb to imitate them), having the speakers close to the wall means that the ear will hear the reflection from the room first, and will either decide that the space is a small one or compromise between the size of the two spaces.

So, from the perspective of imaging, far from the walls is often best.

At the same time, I agree with those who pointed out that you'll lose some bass reinforcement if the speakers are far from the walls, though whether you have too much/too little bass will also depend on placement and your room (because of modes). Speakers never seem to have the best bass in the same place as they have the best image! I usually end up positioning mine for best imaging, but YMMV.

Another consideration is that if you build a wall and make the room smaller, the bass will become rougher because the room modes will occur at higher frequencies. Modes are hard to fix, whereas if you want more bass you can just add a couple of subs (which also have the advantage that you can move them independently, so you can position the mains for imaging and the subs for bass smoothness and extension).