I have a hole around 48Hz in my room. The left rear corner is open, the right rear corner is not.
I really didn't know the hole was there until measured, so not too worried about it for my purposes.
Neither left nor right speaker gives the big hole, but when they play together - with mono bass - the hole appears.
It turns out to be a cancellation.
The Phase measurement shows the left and right become 179 degrees out of phase at the listening position around 48Hz:
You might want to check on this area in your setup.
When the bass in the source program is stereo, the hole can disappear, depending on the different phase in the source.
For whatever reason, i don't seem to notice it, so, ???