Lava lamps. They're completely missing.
No room that size can sound good without a set of lava lamps.
No kidding!
.
Okay, I was kidding.
You don't need a set, one lava lamp works just fine.
I'm right now building 2 wall absorbers right to the left/right
of the place where I'm sitting (that would be where your window
is located).
From the placement of my speakers there SHOULD be no reflexion
from there, but some quick tests showed that it improves the
acoustics.
And when you're there anyway the wall/ceiling corner above the
window has some place for absorbers, too.
Some Ethan Winer entity writes that you can't have enough of
them.
Strange guy
I'd adjust the absorber's thickness according to your 150 Hz peak.
Tom