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.
Move your sub
Caught an Art Dudley video about moving your sub(s) from behind, beside or between your speakers to the seat end of the room. I moved my REL back so they are in the same plane as the listening position. Much better! Try it.