To conclude from such findings on the reason in topology is always alchemy.
Even if we do all the measurements we do not get the information about the reason in topology from this measurements.
If we want to learn to conlclude from differences in sound (or measurements) to a reason in topology, then we must act slightly more systematic. It is simple to define a reasonable procedure, but time consuming to follow it.
Step 1: Make two identical amps.
Step 2: Measure/listen and confirm that they both perform the same.
Step 3: Modify one amp.
Step 4: Measure/listen/compare.
Step 5: Conclude.
If we look to class D and find out if the self resonant modulation acts different from other modulation method, then we would need to build two amps with identical PSU, identical power stage and drivers, identical output filter.... and only vary the modulation method.
I think there was a guy around who did this in order to compare UCD vs. hysteresis modulation... can't remember his name...
I agree exactly, even a semi systematic approach is required in order to ascertain fact from fiction when such all encompassing statements are being made, yet all we've seen here so far has been the total opposite of that, including this recent listening session which didn't begin to compare apples to apples. Strange.
Analogspiceman's previous work has shown it to be a problem, when it is a problem, and anyone can judge for themselves how horrible it is in their own amps. His work has also shown it to be a complete non issue for any reasonable listening level, beyond which I dont' think one can reasonably argue the effect of those last few percent in THD.
Your proposed systematic approach may be used to confirm if maybe what's measurable distortion is also audible. In this case we already know there to be measurable distortion... is it audible? I dont' think so, and audibility will always be just a matter of opinion.
I know the project you speak of. We're unaware of the actual circuit itself, the optimizations and trade-off's either version of module have or haven't undergone, and the hysteresis method of oscillation is prone to its own anomalies...
What you could maybe do instead is just mix a clock in with the input of the UCD, but then you lose all advantage of it being a self oscillating amp, changing the nature of things completely. Still no direct comparison here, but maybe a useful one, you could see which is most offensive between a cheap clock based amp that doesn't account for additional delay, or a simple self oscillating one that's prone to minor non linearity in that last ~20% of range. I kind of doubt you'll come out of that thinking much less of the self oscillating version.