Though available for the past 14 years, there has been very little discussion of pure-digital amplifiers. These are essentially DACs with built-in volume control and sufficient power to drive loudspeakers. One reason for this absence was that very few companies produced them. TacT was the only one from 1998 to 2005 (with their "Millennium" & 2150 amps). Then Lyngdorf came in - but this brand was simply a split from TacT. These co. used the same DSP-algorithms but had different power supplies, etc. so produced different sound. The TacT was favorably reviewed in Europe and the U.S.
Elsewhere, Wadia had their PowerDAC - but it cost a small fortune, was gigantic in size and (probably) due to this, was never reviewed in the press. It was discontinued less than 2 years after it came out.
Sharp and Yamaha had pure digital amps (which were reviewed) but they seemed to have a problem with PCM inputs. As a result, the reviewers used analog inputs, from either LP or CD. This produced an extra conversion, which probably hurt the sound in the end. These were also discontinued.
More recently, many in our community thought that the Behold and Devialet integrateds were all-digital. A closer look reveals they aren't. Behold simply does the DSP in the digital domain, before D/A conversion while the Devialet combines separate analog components in one chassis.
So we're left with TacT, Lyngdorf and two newer entries - NAD an Core Audio. The NAD has been raved in the press, while Core's units are just coming out (and are the first all-digitals to use a linear power supply).
When the TacT came out, some must have thought that analog separates would die off. But few expected these designs would improve so dramatically - look at the latest Ayre, MBL and Constellation gear, for example. The rise of analog switching amps also confused things, as they looked like a better idea than (pure) digital-switching. The sheer number of them for one, but also that these co.'s bad-mouthed their "pure" cousins on technology - and seemed to get away with it. (Their claims that pure-digitals have decimation errors and "can't use feedback" have been overcome, as of late).
Analog interconnects, too, might be gone - despite their dramatic improvement since the TacT.
So - an all-new product category. But it's really our old playback system, greatly simplified. Is the future all-digital ? Or will analog separates continue to improve while their price drops ? I'd say that our separates have a lot of work to do - if they're going to match pure-digital's S/N ratio and cost competitiveness. In his review of the NAD M2, JA stated that it "competed with the best separates". This, at roughly 1/10th the cost.
With pure-digitals (finally) off and running, I believe this concept could be a boon to high-end audio. They offer a fantastic one-box alternate to the high cost, large size and (still) not-perfect-sounding analog separates which have pervaded audio since the 1960s. They make it much more attractive for average folks to become an "audiophile".
Who would have thought that a format war would erupt amongst our (entire) playback system ?!!