Quote:
..My headphones (ATH-M40fs) and my speakers (JBL LSR308's) sound so drastically different ..

I would hope so
My speakers can kick me in the chest .. my headphones; well I wouldn't expect them to do that
They *should sound different.

Maybe describe the room you are trying to fix (?)
22 X 11 seems a bit long so back wall dampening may help with reflections as others noted, but other things come into play; carpet floor? brick wall?
Maybe a picture of the room would help


I do not work here, but the benefits are still awesome
Make your sound your own!