Neil,

I may have to trap above my listening position, but I was also considering
diffusion as an option.

With an 8-foot ceiling you're probably better off with absorption. Note that
the important places to treat in a small control room are the first
reflection points on the side walls and ceiling. These points are not
directly besides you or overhead, but about halfway forward between you and
the speakers.

I've got some bass traps in the corners & on the back wall, and they help,
but I'm still getting some very ****ing annoying low-end buildup - I assume
from the low ceiling.

Just as you can never be too thin or too rich, you can never have too much
bass trapping. What kind of traps do you have, how many, and which corners?
And how big is the room?

--Ethan