Hard surfaces are your enemy. I'd do a thick shag carpet with foam under it as a first step, then large art panels on both side walls and the back wall. And replace the roman shades over the windows with ones that are heavy linen with blackout lining.
Other things I would do - pull the speakers forward as much as possible. And finally I'd re-configure the stereo rack so it's low and wide and not so tall as to affect the imaging from the right side.
One effective low tech you can do is walk around your room while clapping. As you make changes to the acoustics, you will hear the room transform from a 'lively' room to a 'quiet' room.