There's 2 things you can do with an SPL meter, and that's the first: get all your speakers to play at the same level when you run your receiver's test pink noise thru them.
The second thing you can do is to see how your system reproduces the frequency spectrum, by playing test tones. Use your sub-woofer level & tone controles to get the response as flat as you can. If you want to go further, look into getting a parametric equlizer for your sub, and modifing the acoustics of your room.