Musicians

Looks like Justin Bieber’s in rehearsal mode for Coachella ... utilizing a casual furniture run as the perfect space for an impromptu sound check.

Check out this video of JB fully in his zone inside Downtown L.A.’s Deep House Design Store earlier this week ... hopping on the piano and the drums to test them out inside the music showroom after rolling in with his friends.

Witnesses tell TMZ he basically put on an impromptu concert for about three customers and the staff -- hey, a crowd’s a crowd!