A deeper more holographic soundstage.

Several positive comments about Schumann generators, which I have not tried, so I'm intrigued. What should I look for when selecting one? 

It looks like there's a lot to choose from but really its one or two almost identical circuit boards and what looks different is really nothing more than the box it comes in. I discovered this when reading reviews and one that sells for $400 when the reviewer opened the plastic case the photo of the inside shows the exact same circuit board sold on eBay for $12. 

They all use a 5V DC power supply, and many come with a USB cable so all you need is a USB charger and you're in business. Or you can buy the one Synergistic sells, it has the fanciest box of all, and its own power supply, and the highest price tag, but look close wouldn't you know the power plug label says 5V DC. Hmmm....

People will tell you put them 5 ft up off the floor and sure enough the most expensive one Synergistic sells is a tower. But they also have the black box that sits right on the floor. If indeed that's even what these are, not like I know for sure, could just all be one big coincidence. 

Anyway the ones I have are in different locations including one down low right by the power conditioner, and if there is any difference between any of them its really hard to tell. So I would just get whatever's cheap or looks okay to you and put it wherever you have an outlet, sit back, and enjoy. No matter what I can just about guarantee you will be shaking your head trying to figure out what the f is going on, and not caring, because its deep and black and natural and you just want more.

While most all stereo recordings have good imaging left to right, a lot of them are fairly compressed front to back. There was even for a time Phil Spector's famous "wall of sound". Only a relatively small share of recordings have a lot of depth. So first of all make sure you aren't trying to put legs on a snake.  

That said, the first thing that will improve depth is to toe the speakers in so they are more directly at you. First assuming of course they are absolutely equidistant, level, and symmetrical. Then the more in they are the greater the sense of depth.

The next thing that will help create a superb sense of depth is Schumann generators. The dirt cheap circuit boards on eBay work great. I'm running 7 now and probably will add a couple more. Got mine Make Offer $10! No idea what or how these damn things do it but the improvement in space and depth and just an overall more natural palpable presence is amazing.

Another one I haven't tried yet but has a lot of very credible fans is super-tweeters like Townshend makes. They are on my list for this coming year. Townshend Pods, Cable Elevators, tubes, and turntables, whole lot of things upstream contribute, as well as downstream in the room. But mostly its recording, toe, Schumann, super-tweeter.  

Some of these ideas like equipment between the speakers, they might be confusing focus with depth. Yes anything between especially if its reflective will diminish the solidity or focus of placement. But as far as depth itself is concerned, red herring. I've heard systems crammed right against the wall with a rack across the full space between the speakers that threw a stage extending way back beyond the wall. This was in fact the very Linn system that hooked me into analog. So again, upstream stuff like analog and tubes does indeed contribute to depth.