Ok. I will justify my ranking.
I will keep it point form so it is easier to get through and maybe even readable! Lol. If you read through, I hope it makes sense how each items makes the next one better or even possible in some cases. It's a cumulative effort. Many of these factors can be pre-planned, and don't cost money.
It is a case of 1+1=3 when each is added up. If you do all of them, you end up with a superlative and predictably awesome result -because you can make it exactly as you like and learn a pile in the journey!
1. Having symmetrical room for at least LCR channels-even and predictable L\C\R SPL output and frequency response at upper bass frequencies. Channels match.
-even boundary loading and predictable front\side wall cancellation
-even energy and matched timing from 1st reflections
-soundstage has even weight across pans from L-->R in a stereo configuration
-center image stays centered over a broad frequency range. No drifting.
-easier to integrate into subwoofers when front channels are equally room loaded
2. Placing listener on centerline of long axis-same time arrival and amplitude from lateral reflections off of side walls
-1st and 2nd order length room modes are forced to a lower frequency -crossed to subwoofer range
-more flexible seating arrangements when it comes to spacing from back wall in small rooms
3. Correct Listener to speaker and display size ratios-distance between speakers should be roughly 0.85x the distance from the speaker plane to listener plane
-distance from display surface should be roughly 1.15x screen diagonal to seated eyeball at MLP (this ratio allows speaker ratio above to work for speakers to flank screen in AT setup)
-listening plane should be roughly a multiple of 0.32x (large room) or 0.2x (small room) room length from the back wall.
-Front L/R channels should be a ratio 0.2x or 0.32x from the side and front walls but be dissimilar distances from boundaries respectively. Thirds and fifths. Still relevant.
4. Controlled Decay times- targets: AV room RT60 300msec Hifi room RT60 450msec
-shortens the in room decay of transient peaks in program material.
-increases perceived system dynamics.
-improves overall room SPL handling. Can listen much louder without fatigue.
-when the decay of listening space is shorter than that of the program material the psychoacoustic affect is your room is ignored by your brain and the recording becomes your frame of reference for size\space\distance. -incredibly important for multichannel movie and live 2ch recording realism. You are there!
5. Low Noise Floor-improved impression of overall dynamic range of source material
-improved system headroom (need less power to overcome distractions)
-improved dialogue and effect intelligibility at low levels
6. Advanced Tuning Capabilities-Ability to time align channels to each other to match time arrival of direct sound across entire frequency range (multiple drivers) Globally matched impulse response
-Ability to properly delay speaker channels to allow for subwoofer latency and timing alignment
-Ability to properly phase align subwoofer to main L\C\R channels at the crossover frequency
-Ability to sweeten, fatten, warm, or otherwise modify the response to suite taste or room environment
-Ability to add notch filters to reduce the effects of cabinet resonances, room modes etc.
-Ability to compensate for AT screen material over a targeted frequency range if required (perf screens)
7. Multiple Subwoofers -increased overall system headroom and dynamics
-reduction of room modal effects and increased flexibility in multiple listener placement
-improved transient attack (contingent on room decay above)
-improved overall system integration (contingent on proper advanced tuning above)
-removal of any overhang or drone associated with subwoofer "slowness" (contingent on time\phase alignment above)
-ability to truly frighten\startle listeners when called upon in movie soundtracks
8. Having an AT screen-hiding visual cues that confirm where sound is coming from
-spatial qualities are enlarged (width of soundstage is increased to wall boundary)
-ability to use and hide acoustical devices. Bass traps in corners, diffuser on front wall. etc.
-ability to deploy a vertical center channel to match L\R channels
-overall tidy and professionally integrated room appearance. No clutter, subs, wires, or boxes.
9. Wide Dispersion-option to use diffusion instead of absorption at early lateral reflection points
-improved apparent source width
-improved intelligibility and clarity for listeners off axis
-easier to integrate multiple speakers into seamless and realistic soundstage ie. L\C\R
10. Smooth and Neutral Designed Frequency Response-ability to deploy speakers in room with no EQ or tuning required
-greater ability to tune with DSP without losing headroom or creating distortion
11. Room Greater than 2000 cu.Ft.-increased distance from listeners to lateral surround channels. Less hotspotting.
-room for deployment of subwoofers away from front wall
-room for deployment of large front soundstage speakers
-room for false wall employing AT screen
-greater flexibility for larger seating and more listener positions
12. Dolby Atmos\Surround Upmixer
-when combined with a controlled decay time, room dissolves away to create 3D suspension of disbelief
-channels should fire across listening area into absorption to avoid confusing late reflections (of generated reflections!)
13. Outboard Amplifiers-increased headroom for dynamic passages without distortion
-ability to easily turn off L\C\R channels independently while tuning\aligning subwoofers without unterminating
-ability to upgrade in a modular fashion as technology moves onward. ie. AV processors.
14. Having Matched LCR channels-increased dynamic headroom over phantom center
-increased intelligibility of dialogue for listeners off axis
-seamless transition of sound when pans happen across front soundstage
-similar response and boundary loading of front soundstage
15. Diffusion-ability to reduce room decay time without reducing overall energy. Generally.
-ability to enlarge apparent boundary locations (side walls)
-ability to add depth to visual cues when combined with an AT screen. Almost like a Diorama. (front wall)
-reduces the pin point affect of small speakers used as LCRs (front wall)
-creates enveloping ring of sound around listener (rear wall)
