We did an evaluation of class D about 2 years ago. At that time the technology was still immature. My guess is that it is still; the best of the class D stuff, while not bad, does not hold up to the state of the art.
Yet.
It seems to me that tube and transistor technologies are both in a mature state, while class D has still a ways to go, and showing a lot of promise. If you know about price/performance curves, the significance of the preceding statement will not be lost on you! Something that you all want to keep in mind is that one of the bigger promises is that Class D costs about 1/10th that of conventional transistor amps, while the industry gets to charge about 1/2 as much at retail. It takes barely more than the ability to chew gum and walk at the same time to get why they are really here. We have 100 watt/ch. module here that is complete with heatsinks and connectors that fits inside a pack of cigarettes. It is average in its sound with respect to all the class D amps out there, but- it costs $25 in quantities of one... all you add is a chassis and power supply. The incentive to work with this stuff is powerful.
The same sort of thing happened with transistors back in the 50s and 60s- they take about 1/10th as much as tubes to make an amp of the same power, yet the industry gets to charge the same price. Of course the industry had to create a story about how the new technology was better than the prior art, just as was done with the CD. I invite you to consider that the story was made up for reasons other than performance, although as in many fields of human endeavor, performance can always be gained.
Class D lacks many of the artifacts of traditional transistor design, and are already good enough that any designer who ignores that fact does so at his own peril. It appears that class D has already engulfed the common consumer gear market- its ideal for iPods, boom boxes, car stereo and cheap receivers.
A simple tweek for many of the amplifiers is the power supply. Class D being what it is, the power supplies are subject to quite a bit of noise. A very simple means to deal with that very effectively is to use batteries- gel cells- charged by an overgrown wall-wart. Its amazing how easily that can be used to improve things. Like the CD, class D has a lowest common denominator quality. The modules are all fairly close in performance- the big differences are all about the power supplies and noise suppression- at least for now.
I'm really wondering where this can all go. For example, the modules right now are fairly complete units. But what would be possible if the input or even the output could be done with tubes? To understand where I am going with this, imagine an amplifier with a single conventional 300b that could make 50 watts and didn't make any heat. Tubes do not have speed/slewing problems- they can switch at very high speeds. Its the impedance matching issues that slow them down... I predict that in 10 years conventional transistor amps will be all but gone.
