That is boundary affect and moving the sub slightly out of the corner might help cut some of the boominess, even just re-orienting the sub in the corner, ie. flipping it 180 deg. or less, might help. I did this this with a sub, moved it 45 deg. and it made a noticeable improvement in SQ.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70