The main drawback with most PWM switching amps (Which you guys are calling class 'd') is bandwidth and poor bass performance, What you can't be serious?
Pretty much all the switching amps (Ncore, Nord, most Icepower (not all), Nuforce etc are limited to around 50Khz +/-3db which just equates to the slightly more than the second harmonic. Over the years I have come to realise that to have true three dimensional sound with textural layering one of the pre requisites is a decent fifth harmonic bandwidth 100Khz minimum, (though 2.4Ghz maybe a little extreme Mr D'agostino!)
It is not that the switchers are not solid sounding or powerful in any way, they are also clean, dynamic and very quiet, but on the whole they just lack real soul and the ability to really move you imho
They are a couple of examples which are staggering good, however they are cost prohibitive. That said some of the older smaller Tripath modules PROPERLY implemented sounded wonderful other not so.
Again with all circuitry it is a simple exercise in understanding exactly what you are trying to achieve and examining just what influences outside emi/rfi, internal power supply structures, circuit pathways etc are all doing in the final analysis imho.
Well yes much in the same way a bass head is all about lower bass say 90-40Hz that chest thumping club like thwack, not real low end performance that moves you and integrates with the rest of the frequencies in a leaner manner.
Recently we had a well known musician drop off his pair of mighty .5Kw monoblock American pwm amps for repair (power supply board failure due to wrong voltage rated caps (there is a clue) He traded his Linn Klimax twins for them
When he collected I hooked them up to a respectable system and he was smiling from ear to ear, totally convinced that its was top of the pile amplification wise.
I asked if he was up for a Pepsi Challenge, I suggest that a 100Wrms dual mono class a/b amp would make his pair of beasts sound rather thin, nasely and limp wristed in the bass department, plus a much better sense of rhythmic fluidity, he scoffed and easily accepted the challenge.
Mind you after 30 seconds he was somewhat dumbfounded as to why what he had just heard from his pair of prized lean mean switching machines was made to sound very average.
Cost wise they were very comparable, not trying to push an agenda here, merely to agree with a lot of what has been said, a decent a/b with best most PWM switching amps currently.
However depends on what you are looking for, a PWN is at least 85% efficient, much smaller foot print, easier to place at home with the Bula Ballbreaker threatening to place you nad's in a rusty rotavator at 500 rpm.
That said my personal amplifier is a PWM switching amp which I wouldn't trade for a pair of Dartzeel Mono's (they are exceptional with the right speakers) though its bandwidth is the sixth harmonic, switching time is 24ns and I can select a specific algorithm for the switching frequency up to 1.8Ghz (virtually all of the switching amps are around 500Mhz which is about the same switching point of the SMPS they thought is this will cancel out any of the switching noise this is so far outside the pass-band it is inconsequential
It uses two custom built TX's 1.5Kva in a dual rail configuration.
It has wonderfully sweet and lucid mid band, quality tight, articulate accurate bass to whatever the speaker is connected to can muster, including Apogee's (20hm lowest load). A a big open transparent window at the top, it is neither cold nor warm, but just right for what I like.
Clive comments on headroom or power are close to the mark imho