I have nothing in between my speakers in my den but i have a turntable and my rack behind my right speaker, I have been told that placing objects in the middle is bad for sound, but does it matter if equipment is placed behind does it affect sound.
I think it doesn't matter to put things behind speaker if the speaker is designed with air ducts positioned at the front.
Otherwise, hard surface like wood, concrete, and glass may reflect bad effect to sound if placed closely behind speakers which designed with air ducts at the rear.