Ah, yeah, you bring up a good point. I could be wrong, but my understanding is that, in the case of digital inputs, the SHD recieves a digital signal, the DAC processes it and spits it out to analog, the ADC converts it back to digital, the DSP processes that digital signal, then spits it back out to analog. In the case of analog input, the signal bypasses the DAC (of course), heading straight to the ADC.
But, regardless of the veracity of any of that, my main point that the SINAD of the ADC/DSP overrides anything from a higher performing external DAC in the chain, would still stand.
Also, it might be worth mentioning that, while ADC's are part of the chain in most MiniDSP products, SHD is unique in having one that can be accessed for use with microphone inputs as well.