I agree that treating your first-reflection points is a massively important step (and maybe the most important step you can take) when balancing your listening

I agree that treating your first-reflection points is a massively important step (and maybe the most important step you can take) when balancing your listening position, so I'd like to offer a brief correction to your second point, so as not to misguide others who are new to this: Yes, treat your first-reflection points with material that absorbs as much as energy as possible. However, scattering (aka diffusion) should be a waaay lower priority at your first-reflection points. In fact, if you're absorbing with enough mass at these points there shouldn't be much high or mid-range energy left to scatter. And for the low-mid and low freq energy that isn't being completely absorbed you'd need a hilariously-large diffuser to scatter wavelengths of those size. So don't even worry about diffusion paneling of any kind on your front walls. Just absorb, absorb, absorb at your early-reflection points. I personally use several 5.25" GIK Acoustic panels with another 5" air gap between the back of the panel and the wall. At my listening position I have them placed on the wall to my left, my right, and have several panels hanging above my head with as much of an air-gap between them and the ceiling as I can allow to absorb the ceiling-reflections. There's more nuance to how I determine the exact positioning, but that's the general layout. -------- I'd also like to reinforce the idea that for anyone who needs to make critical audio decisions you should absolutely be investing the time, money and effort to get your room treated/tuned to it's full potential. Otherwise it will undermine every decision you make until you do. You simply cannot make good decisions without A) excellent monitoring (speakers) and B) A properly-tuned room. To ignore either of these is to deceive yourself. Consider it your highest priority before you even consider spending money on additional hi-fi audio gear beyond your speakers. If you can't hear your speakers honestly and with minimal interference from the room then there's little point in focusing on the finer details. --------- If you really want to hear how your room is affecting your audio then play a low and steady note (say a 60 Hz sine wave) and then walk around your room while it's playing. Listen for how dramatically the volume of this single note will fluctuate as you change position. It will likely vary from overbearingly loud to near-total silence. Now consider that every other note that you can possibly hear - from the lowest sub to the highest buzz) will also be subject to these kinds of boosts and dips in volume. Then you'll have some sort of idea of what you're up against.