Best Loudspeakers for Rich Timbre?

Well, I do have the dedicated listening room with sound treatment on the walls and ceiling at the first reflection points and corner traps. I have sound treatment on the front wall between my DeVore 0/93's. My speakers are positioned for best sound well into the room. I do however have my equip rack up against the front wall. The prospect of long speaker cables snaking around the corner of the room from the side (I use Cardas Clear, not cheap) is just too daunting. And I have a room with parallel walls because once again, I am out there but not that out there. For those that can afford to configure a room with non-parallel walls, good for them. They are at the margins of the margins. 

While I currently have a dedicated listening room 25X20X12.5 built 25 years ago, I built it wrong with a vaulted ceiling, windows, staggered 6" studs on an 8" plate and dual 5/8" X drywall. Yes, my wife can sleep through 100db sound at night but the inside sound suffered.  My Legacy Focus speakers are 6’+ from any wall and I use 2 pairs of Hallographs for 15+ years and added 32 SR HFTs to correct for slap echo and lack of focus. No bass problem though.  Before the tweaks, my listening room sounded bright and unfocused. Now, resplendent with horn-like dynamics and smooth 35hz to ? extended highs, captivating mid-range.  It didn't hurt to add Omega E-Mats.

I am about to build in my new house, a new listening room but following the principles of first addressing bass absorption where the fundamental notes and mid/high overtones begin (and I’ll save a lot of money).
I intend to follow Dennis at Acoustic Fields recommendation of a dual shell building, ordinary on the outside but with carbon filter panels on a 12" deep interior shell, using birch plywood on the inside surface. Flat 12’ ceiling also treated. This is based on modern sound principals and not my guessing. A turnkey procedure to obtain even better sound than I have now from the start. Basically, the Cardas Golden Rule is not golden and non-parallel walls are so much more difficult to engineer probably using physics. Most audiophiles mistake using home theater, studio and orchestral hall physics and dimensions for use in dedicated 2 channel listening rooms. I don’t want to make that mistake.

Frequencies and Sound Explained #4
Harmonics and Harmonic Distortion

https://youtu.be/FzeZbJceKZE

Interesting Youtube clip on harmonics. I had to watch it a few times before it made much sense.

It seems as if all sound is composed of fundamental notes which also have harmonic counterparts occurring at higher multiples of the fundamental frequency. Its these harmonic counterparts that help identify timbre (tonal colour).

I guess some might find the narrator's findings on tube and transistor amps problematic - but the situation seems even worse when it comes to loudspeakers. It looks like no one even attempts to measure %THD (total harmonic distortion) - not even in high end designs!

As an amplifier designer I can tell you that many speaker designers have very little grasp on the relationship that has to exist between the amp and speaker.
If the speaker is harder to drive, the amp makes more distortion. In particular if the amp is solid state, the distortion will be mostly higher ordered harmonics to which the ear is keenly sensitive as it uses those harmonics to sense sound pressure.
So to prevent this, the speaker should be higher impedance and no weird phase angles. This pretty well precludes four ohm speakers unless they are really efficient! Why four ohms is a Bad Choice if your goal is high end state-of-the-art reproduction is a topic worthy of its own thread!

IOW distortion plays an enormous role, because our ear/brain system simply converts distortion into some form of tonality. So if you want a rich timbre, your best approach will be to obtain a speaker that is easily driven by a tube amplifier, since tube amps make less of the higher ordered harmonics, and thus are smoother. At the very least, a higher impedance loudspeaker means that a solid state amp will make less distortion, and so will sound smoother and more detailed. When 'smooth' and 'detailed' occur at the same time, that's when you are making progress.

I do not think this means any sacrifice need be made to detail and neutrality on the part of the loudspeaker! An excellent example of this sort of craft are the loudspeakers made by Duke (audiokinesis) which are easier to drive due to moderate efficiencies and higher impedances. I can name quite a few others but in a nutshell, the actual tech isn't nearly as important (so long as the designer knows what he is doing) as is the simple drive-ability of the finished loudspeaker.

Ralph; I hope my DeVore O/93’s are on your list of the speakers that meet your preferred characteristics. But be that as it may, I want to add that I made timbre my number one pursuit when I had my last go-round with equipment upgrades. Based on my experimentation, everything makes a difference including-"crap, not again!" cabling. I hate to bring up past beaten-to-death topics but with my ARC Ref6 and Ref 150SE, the cabling between the preamp and amp was critical. The more I invested in cable, the better timbre sounded. I settled on Cardas Clear Beyond. I know you are firmly of the opinion that a properly designed preamp and amp running true balanced obviates the need for elaborate cable, and if you wish to malign ARC as then being sub-optimally designed, I can take that criticism. The very same thing held true with speaker cable. Every speaker cable I tried sounded markedly different. I know all of this makes the average person want to shrug off an impending headache. It is easier to say that this is all a bunch of nonsense. The truth lies somewhere and whether one chooses to search for it or accept what is easy as "truth" is up to each individual.