Subwoofer location/facing question

So for the last few months I've read and learned a lot about subwoofer placement and implemented those ideas which made sense to me. I crawled, heaved, groaned and probably pulled some muscles in the process. I have found thru trial and error the sweet spot or very close to it. It sounds very nice but am sure it could be better.
Here is the question that has not been addressed in anything I have read, which was a lot. 
In what direction should a sealed sub be facing? Parallel to the front wall, parallel to the side wall or facing the prime listening position or other? Seeing diagrams most if not all have the sub on plane with the front speakers facing parallel with the side wall. In my case the sub sweet spot is located approximately 1.5 foot in front of the main speaker plane. Can someone enlighten me which direction the sub should be facing and why? Does a single sub in a system have to be on plane with the mains?
Please enlighten me.
Thanks. PS L & R main speakers are 4.5 feet from front wall, 3.5 from side walls, sub is 5.3 ft from front wall and 2.8 ft from side wall. Mains are toed in 15 degrees. Room size 15x23x8