I had similar issues with a single sub years ago, and did find it best centered as well. Now I have two subs, and there are many benefits. They are inherently less easily localized, so very easy to blend frequency response and volume of my RELs. Soundstage improves.
Single Sub Position
Been moving my sub about to the right or left of rig for months. Still not happy turn it up and midrange gets smeared turn it down and punch just drops. So finally broke down and did the floor crawl thing with sub in listening position. Son of a biscuit it worked. Really didnt want it in this position but it sounds so much better.
How many people end up with it here?