Reading on these, it appears both M51 and C510 can be used as "digital preamp". Both have a output buffer stage, which should be able to drive an amplifier directly (i.e. no need for a preamp in between, unless you want a specific preamp to help shape the sound). There is no analog or resistor based volume control. The "volume" attenuation is done all in DSP by upsampling the PCM signal and changing the original 16/24-bit word to 35 bits and then reducing the actual waveform mathmatically. This is similar to how Oppo does the digitial volume (bit in 32 bit realm). Everything is done in DSP software and output in PWM DSD type voltage injections. There is no conventional ladder-based DAC chip in use here.
I've never heard one of these, so I cannot comment on sound quality.