Invest in a measurement mic @andy2
This - MiniDSP UMIK-2 and use the Moving Mic Method and that should give you an RTA of your space from your seated position and should give you an idea of how your speakers are coupling with the room and how that is affecting your guitar bite and vocal presence loss. I think you'd find the 800Hz to 3kHz recessed when you do this assessment and that should correlate to the guitar bite loss