Four things before you jump into this:
1. Most SPL meters need to have a correction factor applied at lower frequencies. Do you know what these correction factors are for your meter? If it's a Radio Shack meter, you can readily find correction factors on the net or even on this site.
2. Make sure you record the gains that you've set for all of your channels, the main gain, sub gain(s) and all cross-overs. Also record the position of your meter (best to take a picture). You want to be able to replicate this exact same environment so that you can measure again after you've applied sound treatments.
3. If you change your room around, you'll have to repeat all of this.
4. After you've done this, you will wonder what the results mean. Are you prepared to spend the time to educate yourself
?
One last thing. Take an ambient noise measurement before you begin to establish your noise floor. Raise the gain on your main amp to be at least 10 dB above this noise floor.
