"I guess bass sound waves more so accumulate in the lower side of a room, but don’t a lot of these pressure amplitudes reach the upper half?"
Traditional cone based speakers radiate sound in an expanding circle so the sound fills the entire space. This is why people attach absorbers and diffusers to the ceiling, as well as higher up on the walls.
As to whether one can have too many. Bass traps, absorbers and diffusers are like using salt in a recipe. No salt or too much salt both give you poor results.