I finally found a post from Naim discussing the Processing chain. According to this article on Direct digital amplification, the PCM signal is processed by DSP and then sent directly to the output stage. So, it’s apparently different than the standard Class D amplification as their no conversion to analogue. They also have some a special feedback circuit. So, what strikes me from the Naim post is that it sounds very similar.
Clearly, other manufactures think this approach can reach audiophile standards. Naim has said in other posts that standard Class D isn’t good enough. So, maybe there adopting something along these direct digital amplification lines.