WRT the multiwave issue, I don't think the PPP adds enough 3rd harmonic distortion to pose a danger to audio equipment. In fact, that was my main criticism of it, that it doesn't do much. I think what it does is add just 3% distortion or less (maybe it is 1.5%, can't remember, this was actually measured in the review by Martin Collums, who hated the feature). It does seem kind of pointless to start with a typical AC line with 3% harmonic distortion, reduce the distortion back to 0.3%, then boost it back up to 3%. But if, say, you have gotten used to the higher voltage on your B+ rails due to line distortion, the PPP could reduce the voltage leaving you with worse sound, so multiwave brings back the higher voltage that you had gotten used to.
Also, distortion is more complex than a single number. The actual line distortion may include 5th harmonic, 7th harmonic, etc. Very high harmonics are more likely to get past the transformer in a linear power supply. So you have replaced a mishmash of harmonics with pure 3rd. Is that better than no distortion? Well I'm glad to have the choice, and actually I'd also like to vary the amount of distortion up to the 10% that we've heard here as being safe, or perhaps up to 6% just to be sure it's safe. And also lower.
I would agree in some sense the distortion reducing thing sounds overrated, but reducing noise is important, eliminating fluctuations and transients is important, and especially improving line regulation is important. All of PS Audio's regenerators do all these things within their limits. As has been said, the benefit of these changes may not be very important for switching power supplies, but most of the better audio equipment has linear supplies, and most audio power amplifiers have linear power supplies with unregulated HV rails, where the improvement could be noticeable.
I've done a lot of audio measuring, and I've tended to notice a lot of equipment has noticeable noise at 60Hz and harmonics way up. I'm talking about very nice equipment too. Now it's possible most of this is still inadequate power supply design, but it's also possible some of this is getting past the power supply from the line itself, and a clean power line would help.
My audio blog is athttp://audioinvestigations.blogspot.com