The NAD M2 and the Tact amps are the ones I see that are truly "digital amplifiers", meaning that a digital signal is used directly by the amplifier section to send power to the speakers, as opposed to traditional analog amplifiers (pretty much everything else to-date including Class D switching types) that amplify an input analog signal in various ways.
Bel Cantos are Class D but not digital.
Wadia PowerDac does appear to be a true digital amp as best I can tell.
I have no idea what a Firestone Ruby is?
Note that the Tact is actually an integrated amp with digital inputs and digital amplification, not strictly a "power amp" in the traditional functional sense. Many integrated amplifiers these days have digital inputs but apply analog amplification circuits and are therefore not true digital integrated amps.
I do not know if, functionally, there are any true digital power amps only or even if practically there need be with a pure digital amp. Is there even such a thing yet as an all digital pre-amp? I suppose certain kinds of commercial oriented digital processors out there might qualify.
Digital amps, integrated or otherwise, do not seem to be very common yet these days, at least for home use.