Rule of thumb - speakers placed well out into the room will excite less room modes. Keep away from side walls (at least three feet). Sit at around 40% or room length (do not sit with back against wall - very very bad) Subs are best well out into the room against the left or right wall about 3 feet in front of your mains. Try walking around the room while listening - the best positions or setup will have the broadest sweetspot and most even sound.
You will be surprised but rule of thumb works best - it won't give you that "jaw dropping" sound that you get when something is wrong and one note seems to rumble the whole room - so be careful - best setup is actually the most neutral and the least "sizzle" or "emphasis".
Finally use a PEQ or TACT or PARC to fix the worst of the room modes (notch down the "peaks" and leave the nulls alone and above all DO NOT aim for absolutely perfectly flat with thousands of filters - it often doesn't sound right at all - just "flatten" the bumps to with 3 to 6 db of the average and you may find it the most natural sounding - the lower you go in frequency towards 20 Hz the less relevant it becomes to get absolutely flat - flatish between 40 and 100 Hz is most important to achieve!!)
You will be surprised but rule of thumb works best - it won't give you that "jaw dropping" sound that you get when something is wrong and one note seems to rumble the whole room - so be careful - best setup is actually the most neutral and the least "sizzle" or "emphasis".
Finally use a PEQ or TACT or PARC to fix the worst of the room modes (notch down the "peaks" and leave the nulls alone and above all DO NOT aim for absolutely perfectly flat with thousands of filters - it often doesn't sound right at all - just "flatten" the bumps to with 3 to 6 db of the average and you may find it the most natural sounding - the lower you go in frequency towards 20 Hz the less relevant it becomes to get absolutely flat - flatish between 40 and 100 Hz is most important to achieve!!)