Room acoustics

a couple of tidbits from an Email exchange with ASC (Tube-Traps):

1. ona hardwood floor, put a rug down, which should extend all the way underneath the speakers, sticking out for at least a foot in all directions from under each speaker. The rug should also extend all the way beneath the listening chair. This can have a large effect, softening the perceived hardness and brightness of the sound.

2. for glass windows, use the heaviest-available theater drapes from www.rosebrand.com -- even these are acoustically transparent below 1,000 Hz -- but the main components of stereo image formation occur above 1,000 Hz

I have a basement listening room with low ceilings.. two things I've done that have helped reduce peaks, resonance and echos: suspended ceiling with Armstrong tiles and fully carpeted. I'll check which model of ceiling tiles I have, they weren't cheap but they do a great job of sound absorption in the mid to high frequencies

To answer the original question without having to use in room measurements yes acoustic treatments on side walls for early reflections, in corners and on ceilings for slap echo and also diffusers on back wall do make a huge improvement for reducing brightmpness and increasing imaging and overall clarity. You can hear everything way more clearly and focused just using side wall early reflection absorption. I can tell you from experience in my 12 x 13 basement theatre / listening room it was one of the best improvements to sound and coherency I've done. After treating my room I heard instruments, nuances and background vocals I never heard before, everything sounds just more tightened up, better bass and just much better. 
You ears tell your more than anything. 

Hmm, your ears can tell you a lot, but sometimes there are some specific issues that are audible but not easily remedied. These peaks and nulls, which seem to be persistent once the easy room treatments are done, are so specific in their frequency band, that I think you'd be hard pressed to identify and fix them by ear. At least that's my experience. A little bit of data can be useful.
But your mind is still the ultimate decision maker on sonic performance. So it does have to sound good, whatever it measures

My secret methodology for treating room anomalies. You need to find and map out all the sound pressure peaks in the room, incluing the 3 dimension space of the room. The most effective way to do this is using a test tone (s) and SPL meter. I select test tone(s) around 315 Hz or thereabouts since that frequency is very revealing, and effective, but you can use other frequencies, too. This will allow you to find all sound pressure peaks that are say, 6 dB (i.e., 4 times higher!) above the average sound pressure in the room. Generally speaking, these peaks will be found in room corners, at first reflection points and on the wall between the speakers, on the wall behind the listener but also at random and unexpected locations around the room, including in the 3D SPACE of the room. Sometimes right where the listening chair is located! 😧 All of those peaks 6 dB or higher act as "speakers" that interfere with the primary sound produced by the speakers. The objective is to reduce those peaks as much as possible - without producing side effects. But trying to treat the room by ear is a fool’s errand. It’s like trying to solve n simultaneous equations in n + x unknowns. Sure, you can do better than nothing but you will only find a local maximum, not the real maximum.

There are a great number of devices that can be employed to defeat the peaks once you’ve located them. I can vouch for Traps, Hemlholtz resonators, tiny little bowl acoustic resonators, crystals, Mpingo discs, Room Tunes Echo Tunes, Golden Sound’s Acoustic Discs, Skyline diffuser, but not (rpt not) SONEX.