The best soundstage depth I ever heard was 3" single driver in large TL cabinet and Wavac SET amps. Yes, depth was 10’ back or more. Song must be appropriate and one was Michael Jackson’s song with footsteps in it - maybe Thriller.
His room was huge. Maybe 20’ x 30’ and speakers way out into the room. Wall behind was maybe 15 feet back.
The large room, speaker position, along with phase and time coherent speaker were the keys. The first and easiest step would be maximize distance behind the speakers. Not sure of treatment behind the speakers would work. That absorbs reflections.