The sub's minimum distance from the wall should be equal to or greater than the woofer's diameter.
i.e. 12" woofer means at least 12" away from the wall
Run Audyssey with the crossover freq set as high as it goes. Put the volume in the middle. If Audyssey sets it at -15dB, turn the volume down a bit and re-run Audyssey. As long as you're not -15 or +15 it's fine.
If it sets it at +15, turn the volume up and re-run.
I had to do it a few times, but I have a big strong...