Most rooms don’t need acoustical treatment.

Setting up a system in a nearly empty room, once you have studied and obtained a theoretical grasp of acoustics, is a great way to learn the science and the art.  And you don't need to spend a lot of money to experiment ..... piles of boxes, wool and cotton blankets, large pieces of foam can all be used for the experimentation.  Once you've figured out how to optimize to that three-dimensional image we all strive for along with natural ambiance, if the room is to be a listening room, then make the dollar committment.  And you won't waste any, so you'll know exactly what you need.

Once you've got those experiences under your belt, in my experience, it is fairly easy to arrange a living room or family room so that it works acoustically.  If you've moved a couple of times, you can even get to the point of visualizing the acoustics as you lay out the room patterns, and won't have to change much once you have things in place.  Setting up in multiple rooms over time also has the advantage of teaching you what imperfections you can tolerate and which you can't.  Ah, it's good to be an old geezer! :-)

Tooo damn complicated.  Brilliance is the ability to paint complex things in simple terms.  Very few succeed. 
And then enter all the variations of recording qualities that endlessly frustrate.  If only 60s and 70s music were produced presently.  So we fall back and listen to just a few names that bring us comfort over and over again.

Right. Well said. There are simply too many notes. 

https://www.youtube.com/watch?v=Vci4wM6jDDM

I dislike the use of DSP for acoustic corrections for speakers.   Some manufacturers feel that is the solution to mate their speakers to rooms.   I built my listening room to accommodate most speakers, especially those that reach deep bass (my cut-off is now 25 Hz).  My room isn't perfect but it is good in so many factors that I can concentrate on minor tweaks (or upgraded cables/equipment).  Lonemountain is correct-DSP won't make an adequate sounding room great sounding.

It seems that human ears react better to first wavefront of relatively "large" bandwith called a " voice timbre" not to a precise test frequency signal one after the others, like a microphone feeding it back to a correcting program...

Then i succeeded to correct my room with materials passive treatment but mainly with an Helmholtz "mechanical equalizer" calibrated by the the first wavefront of sound created by the specific timing of the frequencies of early and late reflections adding themselves to the direct waves from the speakers and coming also from the waves modified themselves by their near 80 crossings of the different zones pressure of my room each one second...

This mechanical equalizer is made, not of bottles like the original one, but of tubes and pipes, sometimes one inserted in an another thinner one, with a short or longer neck(various type of straws) which length i tuned with hearing and listening experiments... It takes me a week and perhaps 50 hours to tune the 22 tubes and pipes....Location is important... All refining parameters were dictated by sound like a piano tuner use his ears and implemented by cut in section of various diameter of straws inserted in one another....

I also use golden section in 3 set of 3 pipes among all the other singular 13 tubes and pipes... My longer tube is 8 feet long under my 81/2 feet ceiling.... Correcting these 9 pipes together was the more easy part , they were very impactful ...I also used 5 smaller pipes of various size near the tweeter of one speaker (3) and near the bass driver of the other speaker (2) to create a more audible first wavefront signal without changing the basic identical parameters of the speakers but only their response to the room but not their frequency response more their first wavefront timing response... It worked marvellously.... It was my adding modification piece in relation to speakers, to the original mechanical equalizer before the invention of the speakers itself....

I say all that because Helmholtz was able to set a room without DSP better than DSP... WHY?

We forget that, and we called DSP a progress and it is one indeed for "precise" measurements of a tested frequency in relation with a "precise" location in millimeters...

BUT we forgot that human ears is used to hear not a frequency alone, but a wavefront constituted of many frequencies together, usually a human vocal timbre, and we forgot that a room must be tested for itself not from and for a precise location in millimeter mainly ... And a room is not a set of passive bouncing walls waiting for the tested emitted frequency anyway, but an enclosure for the human voice, and an heterogeneous set of variable presssure zones, with the tubes and pipes being some of them...

The mechanical equalizer work on all audible frequencies not only on bass like many think, and they work at same time from near listening location and regular location in the room...not from an ideal "imaging spot" that is never an ideal spot anyway, the ideal spot is not made only for "imaging" but also for the " listener envelopment" factor ....The belief that near listening shield us from the room problems is completely erroneous in "small" room...

DSP is and could be a marvellous tool, but those who think that he can replace human ears must wait that new learning neural algorithm implement it in  an A.I. expert system... Soon it will be done.... But the cost will be high.... In few years tough we will use it...

For now my ears is the main tool and it is enough...

It is a good thing also to learn acoustic by ears not by equations mainly....

I am not a scientist at all.... All that is my experience only and could be wrongly explained .... But here we describe our own experience and i tried....