As far as I know, the TI Equibit technology used by Panny and Tact is the only 'direct digital' approach for PCM audio (ie normal CD or DVD). The other approaches such as the Tripath and IcePower all are really variations on conventional analog technology that require analog signals for feedback, and so the idea of 'direct digital' interfaces doesn't make any sense. Tripath is supposedly going to offer a direct digital interface to their chips, but AFAICT this will just integrate a D/A onboard, which helps with cost but not necessarily quality.
The Sony DSD amps are somewhat different in that there is also no conversion, but the DSD signal is already effectively a pulse modulation. Conversion from PCM to DSD is pretty much exactly what bitstream D/A converters do, so you are still faced with a form of conversion in this case.
All in all, though, 'staying digital' is merely an implementation detail - you shouldn't really evaluate a particular amp by whether there is or isn't a conventional D/A stage present. As has been pointed out - it's all analog when it gets to the speakers, so there is ALWAYS a D/A conversion of some form happening. Don't get TOO hung-up on where it occurs.
