@sbank I believe you stated that the Backert Labs preamplifiers are not fully balanced, this is not true, they are fully balanced where there is an XLR connection, both output and input. The Rhythm has two XLR fully balanced inputs while the Rhumba and Rhumba Extreme only have one fully balanced input. All units have one fully balanced XLR output.
Concerning the OPs question on creating depth I believe one should begin by looking at the room acoutstic / layout and the positioning of the speakers. 30 inches out into the room might not be far enough, particularly if measured from the front of the speaker and if the components are set between the speakers. It is not apparent to me from the post how the room is laid out nor the speaker positioning within the room. Furniture can easily get in the way of creating the perfect sound stage.
I have two Backert preamplifiers, Rhythm and Rhumba Extreme and they both help create a wonderful sound stage which includes wonderful depth, however my speakers are 54 inches, measured to the front baffle to the room's front wall. My rack, which is between, is extremely low. The top of the preamplifier is no more than 26 inches above the floor measured to its top. I also have no furniture other than a sofa with a small coffee table in front of the sofa so the speakers really float freely in the room.
Prior to moving onto new preamplifiers in hopes of developing more depth with in your sound stage I would play with the room and speaker position. Certainly the first thing I would do is pull the speakers further into the room and make sure nothing will interfere with the sound as it leaves the speakers and heads toward your listening position, nothing around them to block the creation of the sound stage. Sound absorbing panels placed at the first reflection points and if your room is on the smaller size some bass traps will greatly enhance your sound stage.
Another area to explore is speakers. Certain speaker designs are much easier to create pin point instrument placement within a sound stage. You should be able to create the illusion of musicians being placed beyond the front wall, which is what I believe you are after.
Good luck.