What affects front to back depth in room/ system?

It’s in no way a matter of cabling and tweak products such as vibration absorbers, contrary to what the either delusional or perfidious dealer suggested here. Yeesh…talk about misleading.

The primary factor is how much space you allow between the speakers and room boundaries. Additionally, bass smoothness, proper room treatment for ideal decay time, the phase of the recording, and overall speaker performance also play critical roles. All the rest affect perceived depth by far smaller margins.

I’ve setup systems that can create incredible depth for relatively little money, so it’s not so much about how much you can spend, but high quality speakers are a must. 
 

I see quality loudspeakers there. Wide-angle distortion in the photo makes it difficult to estimate the ratio of tweeter-to-tweeter and ear-to-tweeter. Some say that these should be equal (an equilateral triangle); however, most great-sounding systems I've heard have the speakers a little closer together. Tweeter-to-tweeter distance is ~85% of ear-to-tweeter.

I agree with others that for most loudspeakers to create a perception of depth, you're going to need to pull the speakers well out into the room…at least 4 ft from the front of the speaker to the wall behind. Sometimes further is better. It also helps for there to be nothing between the speakers. Bonus points for having nothing inside the triangle formed by the tweeters and your ears.

Good luck. It's something we're all chasing and have achieved to various degrees.

It’s all about setup. There are several excellent books on setup. Buy one, read it , learn how to setup your room

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.).