Speakers with no rear wall, better or worse sound?

I have been in a debate about speaker placement. I was proposing that having speakers in the middle of a long space (~30 foot long room) would be preferred over having speakers about 3 to 4 feet from a wall behind the speakers. I would think that having little to no wall reflections of sound from behind the speaker would be preferable. The wall behind the speakers is currently about 14 feet back. Another opinion suggested that we should build a wall 4 feet behind the speakers to get the best sound. I was thinking that the reflected sound from this new sound would mix with the speakers' direct sound and erode the quality. Other views? We were considering some renovations and I do not want to go through a significant change to our home and erode sound quality.
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).
Some well intended but misinformed suggestions here with reference to the goal of Avalon Acoustics. I think you already understand the basic principals of thirds as described in the Avalon manual when considering your own room.

As simple as it's described in the manual, if like me, you're dealing with an irregular shaped room it will take a degree of trial and listening. The right recordings and speaker mobility during this period of adjustment can be your biggest asset.

After months of this I was sure I had found the magic placement only to have a friend who is very experienced with Avalons separate the speakers a few more inches and add 1/4" of toe-in and bingo. The sound stage seemed to surround as if the timing of reflection found the listening position optimally. In the end it takes someone who has experience with the particular speakers potential to wring the most out of your room.

The affect this had on the speakers bass was also a result of timing and not about an increase or decrease in bass. In my case (or my taste) the addition of two small Velodyne subwoofers to blend seamlessly and control the lowest octave also had the effect of adding size to my room.

Good luck with it.

I don't know whether it's physics or psychoacoustics, but the only way I can perceive soundstage depth with any real effectiveness is by positioning my speakers as you suggest, Hickory. Bass response is fairly easily addressed by changing the proximity of your head to the wall behind you. That does pretty much the same thing as changing the proximity of the speakers to the wall behind them.
Thanks to all for the insightful suggestions. At a minimum, I have a few other considerations that do not involve major construction. I do not believe that bass in the current set up is lacking, so building a new wall may mess things up with respect to null and re-enforced modes as suggested by Josh358. Also, for Vicdamone (PS. I have a couple of records of the 'original' Vic Damone) I have spent countless hours playing with toe-in with my Avalons and with great benefit. I used to be disappointed in the soundstage width, in particular, which is currently much improved from all of the tweaking.