Your layout reminds me of this even smaller build.:
How tall is the basement? If you have decent height, you could raise the whole floor, to be at one level.
The sump pump could be given a hatch cover for access while the piping be re-plumbed to swing over.
Then the whole raised floor could be treated as a full range bass absorber. You could even build
the raised floor as two separate structures so you could have an invisible sand filled front stage for your speakers
to sit upon. If the stairs are basic builder grade basement stairs, you could create a landing at the first step height,
and use this as floor level for the theater, if you could give up 7.5 to 8.5" in height in the theater. This could also let you
move the doorway further from the screen wall and maybe create some speaker space at the entry. And a door would give
you a symmetrical side wall.
What's that white cap on the bath tub overflow pipe?
I think you have a very workable space. The choice of projector and it's throw distance is what you need to be careful with.
Benzjas.JPG 63k .JPG file