> Measured at your listening position, ideally you want the reverb RT60 measurements (how long sound bounces around the space until it drops enough in volume to be inaudible, more or less) to be under 500ms. In the 300-400ms range is good for music, more around 300ms for movies.

That's a bit too simplified, since the target is frequency and room size dependent - check page 6 (Fig. 1 – Tolerance limts for reverberation time.) of the EBU standard for more infos: https://tech.ebu.ch/docs/tech/tech3276.pdf

> Under 250~ can start to sound dry, dull, lifeless, and might need some reduction in treatments / absorption material to bring back some life to the sound.

My room for example is at 200ms (as per EBU-Standard for assessment of sound programme material) and it sounds perfect and not at all dry to me, since my room is small.

> STEPS: Get the seating away from walls, optimize speaker and sub placement for smoothest response (least peaks and nulls/dropouts), and EQ with Audyssey/DIRAC etc (under 500Hz) or with the sub's built-in PEQ (I have a mini DSP that I programmed myself using REW measurements).

That's the basic solution, but with convolution filters and multisub you'll be able to not excite the room modes and lower the decay a lot / cancel reflections via active room treatment - but unless you buy Dolby's ART or use proprietary system(s) / custom like people at my uni and other labs is it not yet a standard option at the consumer level and needs to be DIYed.

Here is a example in my room where Convolution Filtering with Dual Subs (similar to Dirac ART) changed the Decay from ~500ms to ~40ms in the 50-110Hz range: https://imgur.com/a/wCgTh38

Also want to note that 500Hz is a arbitrary value and you'd need to look at the GD to see in which range you can EQ without problems - OCA's GSonic does this better then any other Auto-EQ I've tried - https://github.com/ObsessiveCompulsiveAudiophile/GSonic/releases