Absorption, Diffusion or a combination of it all?

I see advice all over the map here yet nobody condsiders the size of the room? It's not one size fits all!

OP, diffusion is great but unfortunately not of much use in a small room simply because you need to be seated a minimum distance away for the effect. Situated close to your ear will just not do.

My suggestion is to at least heed the advice to measure the room yourself. That way you can ignore the noise being made about creating a room that is dead.  It has been mentioned to get GIK to 'analyse your room' and then buy their products. Well they are in business to sell stuff and will, as has happened to some I know, oversell if you keep buying. 

To measure: Install REW, it's free

Buy a microphone: for the cost of a dinner.

This will provide the tools to achieve the ideal response in your particular room. Tables online will supply the target time required for the sound to decay, across the spectrum by a certain amount, this amount also found in the tables.

What I can assure you is that with the confidence you gain with understanding the requirements for treatment, and can measure any changes from adding traps or panels, will be the biggest step towards an immersive and realistic sound. Ideally the treatment will essentially largely remove your room's sonic imprint and transport you to the venue.

No electronic component exists that could correct the acoustics of a room regardless of cost. The recommendation to 'just get DSP" to sort out the problems can not sort out the damage that strong early reflections cause. How can any electronic gizmo know how much decay to apply and at which frequencies to apply it?

Should you hire 'professionals' to do an installation you can of course measure the results to ensure they do not reduce the decay time beyond the recommended time, which is generally 300 to 400ms. Overdoing the absorption is what renders the room being too dead. You newfound skills will allow you to prevent this.  It will also help you find the best position for the speakers and a sub or three.    😀

The subject is fascinating, understandable and most importantly, essential for great sound. I repeat:  No $5000 cable nor $10000 amp can do anything to tame the elephant.   But you can  😎

Love this topic as it’s not as common but extremely important.  Working with the folks at GIK (or other acoustic expert) is key.  REW has been a very helpful tool along with my ears, and the USB mic was a worthwhile investment as I’ve used it multiple times over my three year journey in treating my room.  I cannot recommend it enough. And the GIK folks will use those REW measurements to help diagnose your room and suggest treatments.  My listening chair is only 2’ off the back wall so pure diffusion wasn’t recommended.  I’ve read elsewhere that pure diffusion on the back wall isn’t beneficial unless your listening chair is 6’ or more away from it.  GIK  instead suggested their combination diffusion/absorption Alpha series panels, as I had low frequencies that needed to be tamed along with me trying to make my room sound bigger,  My last and final addition was adding 3 x Q7D diffusers to the front wall and a pair of wall hung monster bass traps directly behind the speakers.  My small square room has never sounded better.

Having a good mix of absorption and diffusion is certainly essential. Also consider DSP based room correction technologies which operate in the time domain. Dirac Live helps listeners correct for one of the weakest components in the audio chain, the listening room. Dirac Live not only corrects the frequency response, but also the impulse response of the loudspeakers in a room, yielding improved imaging and timbre, better clarity, tighter bass, and less early reflections, as well as reduced resonances and room modes.

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If most listening is below, say 85 db, are room treatments still as important?

85 decibels is a sound level equivalent to that of a food blender. Uncomfortable and only tolerable for a short time. Most listening levels might not even be that loud.

Having said that, peaks can still get fairly high above the general listening level, so treatment is still wise.

Maybe not as important, but crucial.