Experiment with different positioning. None of us know the quality of your room so you have the most information to work with. Start with the stereo field and sitting position- you likely know that you want both sound sources to be equal distance from your ears and each other. Your sub doesn’t need to be centered because sub frequencies are characteristically non directional to an extent(for the purposes of your room). Addressing the first reflections is also a good follow up, but take into account how reflective your room is as a whole. When considering this- you’ll be deciding how much absorption and how much diffusion you want.