ZD, I only "measure" things with my ears, thus I apologize in advance for the unscientific and somewhat subjective nature of the following....
On the subject of treble intermodulation.... Frequently, on multipart string music, you may notice that there is a range of chorded notes, approximately in the 3rd treble octave, that have the habit of sounding rather harsh, distorted, and metallic. What I always look for is ways to make those notes sound like "music", without resorting to filtering effects.
On the subject of harmonic development.... if you listen carefully to piano music for example, you may hear the fundamental frequency, but also a set of harmonic frequencies that rise, sustain, and decay on their own terms.... this complex and evolving harmonic resonance is what gives an instrument some of its individual richness, or.... palpability.
My experience is that a PFC-based DC front end for an autoranging SMPS can increase an amplifier's ability to expose the complex behavior of independently decaying harmonics, and control a fair amount of treble harshness, without ever being perceived as a band-pass filter.
For example, integrated PFC rectification is one of the main differences between the class-D Rowland M501 monoblocks and the class-D M312 stereo.... Yes I know, there are other factors involved, such as higher quality transformer coupling etc... but.... M501 is a fun amp with a lot of power that sounds a little "matter of fact", while M312 sounds much more musical and emotional.... M312 Was my very favorite power amp prior to my adoption of the Rowland M725 monos, which while being based on class A/B design, are still fed by a PFC-based DC front end. Back to M501, the monoblocks can be front ended by a PC-1 PFC-based rectifier on each... Owners that have used M501 in such configuration claim that they rival the overall musicality of M312.
Warning, unlike Furman Elite, PC-1 transforms 120V AC into a 220V DC current... So do not feed it into any device unless the target component has an autoranging power supply, or it has been switched to 240V operations, lest.... you will be regaled by a cute puff of blue smoke and a slagged component.
Guido
