What affects front to back depth in room/ system?

Note that unless you've never actually owned a stereo system or you have no faith in your own taste, the proper setup is what sounds best to YOU.

I went on the search for greater depth and have concluded it is a bit of a myth.  My speakers are 4' off front wall and 3' off sidewall.  My side reflections are treated. I have a fabulous sound stage and very holographic sounds.  There is depth but not to the degree some describe (i.e. behind and outside the front wall.).  

While it sounds counterintuitive, side wall reflections impact the width and depth of your sound stage more than I ever imagined.

As you consider room treatments, read a few articles written by Art Noxon, owner of ASC (Acoustical Sciences Corporation) Tube Traps. 
 

I like his tube traps because you can literally “dial in” the soundstage by simply adjusting the amount of diffusion and absorption you need. 
 

Use a mirror to find your 1st and 2nd side reflections, and watch your soundstage adjust to meet your needs. 
 

Hope this helps.

labpro is correct that sidewall reflections affect image depth...all other things being equal, by moving speakers closer together you can increase soundstage depth but at some point it's at the expense of sounstage width. It's an iterative process, and almost every room is different...

It's probably already been said, but FWIW real 'depth of image' is captured in the recording process. If it ain't there it ain't going to be anywhere.

In a carefully set up two channel system all you get is what is on the recording. There are many variations of what folks think is soundstage mostly depending on how they use their room in their set up and think what they have achieved is really the 'depth of image' so often referred to by folks, it's not. That sound is only the result of management of the rooms construction, size, speaker placement, reflections, and nulls/nodes, etc. 

If you want to see how your system reproduces true depth of image get yourself a recording that has it in the first place. I always recommend Opus 3's "Depth of Image" on an LP if you can use it or CD. IF your system is properly set up its sound will blow you away. It's just amazing! This recording is a compilation of classical, jazz, pop, taken from their recording of such music from their other recordings. Their mic set up is simple and 'correct'. The manual will discuss all of the cuts and tell you what you should hear - if you can't detect all that they tell you exists, then you know that you need to adjust your setup to get there. High quality equipment may help some, but a reasonably good system can get you there.