Move your sub

Your optimizing the bass time arrival if subs near plane loudspeakers occupy you could also place behind listener or on sides if physical distance is near same as seated listener is from both speakers and sub. You could keep subs physical distance correct for time integration and pursue boundary re-enforcement that most subs require. Or use multiple sub systems this gives better measured response in most rooms lower distortion increases system output.

Placement depends to some extent on what subs you are using.  With the Vandersteen 2wq subs that I use, I need to keep them near each speaker as the subs reproduce frequencies high enough to affect imaging.

Proper sub integration starts with the room position that gives the smoothest response as measured at the listening position.  For my sub it happened to be just outside one of room corners in front of me.  
From there you can adjust continuous phase (a required feature), polarity (a required feature), crossover and level to phase and time align with your main speakers.  The result is seamless, non-directional perfect subwoofer integration and optimized power response.  

@miner42 

https://www.youtube.com/watch?v=k3hRbm7KU1U

Bass actually is omni directional to the degree that low frequencies sort of crawl along surfaces and react to room corners, etc. If you walk around the room as Dudley suggests you understand this fact, not negate it. I have one sub right behind my left speaker, and another smaller sub a couple of feet away from my right speaker against a wall where I can easily put it in the window to service my deck sound needs when appropriate. The second sub sort of tempers the other one to smooth out the whole thing ("swarm" sub systems do that well I've heard). I actually would try Dudley's method if REL decided to offer their "Longbow" wireless system in a version that could be used with older models…I mean it's simply transmitting to something in the High Level input at the sub…come on REL! This aside, my 2 subs sound great as is.