Recently there have been several positive reviews of the NAD M2 Direct Digital Amplifier (white paper). It is the first of its kind, and I expect many more amplifiers like this will follow.
Today most audio sources are digital: CD/SACD, DVD/Blu-Ray, HDTV, PC/MP3 server, etc. These signals are passed through a digital-to-analog converter, have volume control applied, and are passed as analog signals to the amplification stage. Even current class D amplifiers accept an analog signal, convert it to a PWM (pulse width modulation) digital signal, and use that to generate the final amplified analog signal sent to the speakers.
The NAD M2 is unique in keeping the input digital signals in the digital domain as long as possible. It accepts PCM (pulse code modulation) digital inputs, applies volume control digitally, and converts the PCM to PWM for output by the class D amplifier. Keeping the signal digital as long as possible eliminates all the possible noise sources in the redundant digital-to-analog-to-digital conversions, and the lossy analog connections between source, pre-amplifier and power amplifier.
(Yes, I deleted my previous audio post as I realized I was spreading disinformation. I am currently fascinated by the topic, and will keep trying.)