what kills soundstage depth?

Hi folks. Thanks in advance for your thoughts.
I am getting nice solid imaging from my current set up, but it is 2 dimensional. No depth. And I'm not sure why. my hunch is speaker placement or lack of room treatment. Any thoughts would be helpful. Also- a suggestion for a good recording for testing soundstage depth would be helpful. My taste in music is pretty lowbrow and it might just be they way stuff is recorded.
Thanks!