Just as an opposing point of view, I'd like to say that I have done a fair amount of comparison evaluation of Class A/Ab and Class D amps.
The switching power supplies are another story all together as I feel they don't cut it with powerful subwoofers. The heavy toroidal type transformers with a lot of reserve capacitance just has the massive headroom for large dynamic power demands. Take a look at the Bryston 28B3 monoblock amps (Google image these monsters and you cannot believe the power supply section), They are pumping out around 1800 watts into 4 ohms with what I am told is almost unimaginable dynamic power when under heavy loading. I have not had the luxury to try these out yet, but I'd love to.
I have a nice Parasound A51 amp which is Class A operation up to around 10 watts output and then migrates into a Class Ab amp when the power goes up as it can do 400 watts per channel into 4 ohms. It's a great amp, very smooth and musical. I was sent one of their new design Class D amps to try out and I must say that on the lower end, it was impressive with its 600 watts per channel and gut wrenching bass response. I did not like it much on the upper mids into upper range as I found it harsh on my ears after a period of time listening. But it is not an expensive amp and it uses the typical off the shelf Class D power modules which many companies elect to go with.
From there I installed a pair of Anthem Statement M1 monoblock amps which are gorgeous Class D slim pancakes vs. the A51 Parasound A/Ab amp. The M1's are connected to my main panel via dedicated 240V 15 amp circuits, one for each amp.... and running them on 240V vs. 120 V 20 amp lines made a huge difference in performance. At 4 ohms, these babies pump at a minimum of 2000 watts per channel and all I can say is that it was a major WOW moment when I turned them on with some music I am very familiar with. Massive low end, super silky mids and smooth/articulate Upper range. It was almost too good to be true and I wanted to hear how they were on me after a night of long listening.... just fabulous, absolutely no harshness at all. These amps have liquid cooled output devices and very proprietary feedback loop design along with an excellent power supply section. There are no off the shelf Class D modules in the M1's they are all Anthem of Canada design and built in Toronto and expensive at $7K.
Now after a few days of use, I decided to swap out to my trusty Parasound Halo A51 A/Ab amp just to get back into Class A again. After about 1 hour of use, I removed it from the rack and went back to the heavenly M1 monos.... no contest! The Class A amp was just blah and lacking in articulation vs. the M1's in Class D. The M1's are still here, I didn't use the A51 for about a year or so until I decided to use it for a surround system where it is doing duty as the center/surround/rear speakers which it does well for. But for critical listening on two channel stereo vinyl with my VPI Ares3/SME Series IV tonearm/Ortofon A90 MC, not much will come close to the M1's. Again I have not had the luxury to try the monster Bryston's, but at $30K for a pair of those monoblocks, I might be waiting.
