Ok great, you know how to use REW. Take your spl meter and lay it on the baseplate with the mic towards the center of the driver if possible - if it's too thick to fit between the bottom cap and baseplate, then lay it on the ground right next to the baseplate. If you don't have carpet floors, use something like a mousepad to keep the meter from rattling.
Now take a FR sweep.....it doesn't even have to be a full sweep, 10-50hz will be plenty fine. This should reveal the exact tuning point, as the output from the driver will overpower that from the ports at such close proximity. You'll see pretty flat response, then a dip, then one last hoorah. That valley of that dip is the tuning of the sub. Here is an example of a close mic FR of my sub:
Tuning ended up at ~13.5-13.6hz. Make sure you keep all three ports open. You can then try it with one closed and two closed and see where the real tuning ends up in those configurations.