What affects front to back depth in room/ system?

I've been moving speakers around for a while now trying to maximize their placement for a happy balance of soundstage width, focus of center image, vocal height, instrument placements, etc. I want to get the speaker placement settled before acoustically treating the room. The room is 15x20 with 8' ceilings. Speakers are setup along the 20' wall. I'm pretty happy with most aspects of the sound, but what I can't seem to figure out is how to improve the depth. Honestly, I'm not sure if what I'm after is attainable to begin with. Is it possible to have depth that reaches the listening position in a 2.2 channel sound system? The depth behind the speakers is great, just not much in front of them- unless it's one of those songs that has a part where it has that inverted phase trick. Then it washes over me. I want that all the time. Any feedback and advice is appreciated. 

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Your perception of depth of soundstage is most likely negatively impacted by the first reflections (side walls, ceiling). Also, adding bass traps will help solidify the soundstage. When your brain needs to process sounds bouncing off every reflective surface, it will have a tough time realizing the stereo image. 
Easy way to test it is at very low volume where the reflections are minimal, does the depth increase?

Let's consider some of the basics of perceived soundstage depth.  Sounds that seem way behind the speakers have reverberation which is either natural--from distant-miking--or artificial.  Sounds that emanate from the plane of the speakers are typically close-miked without much reverb.  To have sounds in front of the speakers in a stereo system would require that the phase was reversed.  Or maybe it can be done with a surround system.

With regard to southern California dealers, I thought Alma Audio in San Diego did a nice job of setting up their Boenicke W8 speakers to create a sense of depth. The sound was very enveloping. Nice looking speakers that sound excellent, imo.

@veerossi adding to my earlier post. Now that I saw your system….very nice! But I’m afraid, in addition to room acoustics, lack of preamp is contributing to the soundstage limitations you’re experiencing.