As far as sub location goes, you will not have too many choices due to interaction between room and lower frequency. The place you find sub best suited (in the cavity or ceiling) might not necessary be the best location for the sub

The best you can do it is to place the sub where you be sitting, play a song with a constant bass notes (turn off your main speakers), and crawl on the floor nearby your front speaker and listen for the bass response from the sub. Bass notes should be distinct and not boomy when you find a suitable place. And then place the sub there or close to its vicinity.