The issue is, we are not talking about the "pure" speaker response, but the "in room" responseI feel your pain, but I view the situation differently. I believe, to a point, that the best sounding speaker after correction is the best sounding speaker before correction. Other factors are the speaker’s driver layout and the port design; or lack thereof. I have never heard a single uncorrected stereo system that could compete with a properly tuned, corrected system. Right off the bat, unless the speaker uses a horn or waveguide that recesses the tweeter into the cabinet, the tweeter and midrange will not have anywhere near perfect time alignment no matter how well engineered the crossover is. Besides impulse response correction or a DSP with delay; a plinth or stand that angles the speaker are the only other honorable mentions to address this issue.
The best systems I have ever experienced all shared a common element: externally powered active speakers; zero passive crossovers and no internal amps. The benefit is being able to DSP each driver individually. REW let’s you measure a driver and set the PEQ target to compensate for the crossover filter slope. That way, you can focus your PEQ on the area near the crossover, probably the speakers weakest area for even response, helping to blend that driver with the other drivers. Once you integrate each of the active drivers with gain, x-overs, delay and PEQ, a lot of DSPs will let you PEQ or Dirac on top of that, but some won’t.
The best case scenario of an uncorrected system is to have the size and design of the speaker perfectly match the environment. The closest I have ever gotten was with Focal Chorus 705v. The inverted tweeter dome, small 5” woofer and front port design worked excellently in a medium-large size room that was 12x24x10…but people want the bass from their Kanta 3s to sound just as smooth and even in the same size room.
While not completely popular, my opinion is the goal should be flat frequency response from 20-20k. The lone exception to this rule is big bass, but I never boost bass. Big bass should only come from big amps and big woofers.
Dirac has the unique ability of essentially unlimited filters because of its mixed-phase design. Dirac is not limited to IIR and PEQ filters like Audyssey. People talk about which target curve is the best. I do not run a target curve in Dirac. I delete all of the “control points” and let it do it’s thing. The result is even response from 10hz-24k, as long as you have the speakers to do it. A lot of “full range” speakers are really only useful from 40hz to about 18k and room correction can not improve that. The most important thing I have found is that the results heavily depend on the calibration microphone and the accuracy of its cal file. UMIK-1 was a monumental step up from the mic that came with the M33.
I use the EQ function in REW to interpret the graphs. If REW can automatically determine the target and generate PEQ for 20-20k without any warnings about the target being too high or low, that’s a pretty flat speaker to begin with. I’m not an engineer, so I can’t really interpret the graphs visually, I also don’t have the background knowledge of what to look for in the graphs. By tweaking parameters in REW EQ, I can easily gather data I can understand. If I run REW EQ at +/- 1db and it only generates 1-3 PEQ plots for the entire 20-20k spectrum, I know I’ve got it pretty dialed in.
Dirac does do some impulse processing - but it's major impact (along with audyssey) is in tuning the frequency response ie: the Target Curve.
And the Target curve includes room absorption, reflection, etc...
Optimal "best sounding" room response target curves, from flat F/R speakers are in most cases tilted - ie: higher in the bass and lower in the highs - setting the room target curve to flat (as you propose) is likely to result in a very very bright sounding room.
Whether or not you like it that way is a different issue - but it is certainly a very unusual setup - and not reflective of what most people would consider optimal. (The Harman studies identified an optimal curve preferred by 60%+ of the population leaving 30%+ with different preferences!)
Personally, I find most Harman curves too bass heavy.... I am still experimenting with my Target curves, but I tend to prefer the Dirac default curve bass to the Harman boosted ones (the default dirac still has the tilt, the bass is higher than the highs, but it is a straight line tilt, rather than a further boosting beyond 100Hz of +4 to +10db (so circa +2db bass maybe...)