There is a guy on the elemental designs forum that has a room just like that. He placed black acoustic panels along the walls and each slanted ceiling, said it helped, but before it was not bad.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85