8341A flush mounting box is designed to avoid reflection from the wall behind the speaker. Benefits are the lack of cancellation dips caused by rear wall reflection and increased bass level due to the 2 pi radiation space. The increase in bass level can be compensated by GLM without changing the increased SPL capability.

It is good to keep in mind that reflections from other surfaces, like the floor, can still cause dips in the upper bass / low midrange region.

https://www.genelec.com/accessories/8341-450b-flush-mount-kit