The materials and kind of wall construction will be crucial to making this work in my experience, regardless of speaker. You need to avoid unwanted vibrations from the wall itself that will muddy or enervate the sound. A modest wall treatment might be the best investment you can make. I've had some speakers as close as 12" with good soundstaging and bass balance. I'm no acoustic expert, but if you are interested in this issue, let me know and I can share my experience treating the wall.
