I think this is reasonable. If he had two miniDSP's (one for the subs and one for the mains), he may be able to get equal or better results. Of course, then you have to consider whether to buy two additional miniDSP's for the center and surrounds.
As far as DAC and bit rates, Set your crossover to 100Hz, unplug all of your speakers and listen to your sub(s) alone and tell me about all of the subtle details that will be lost with a slower bit-rate.
This might be an issue in the mid-range where our hearing is most sensitive (I have no idea, I have not researched it), but I cannot imagine it mattering for the bottom end!
However, as Pogre suggests, start a new thread targeting specifically what you are interested in discussing.
This thread has a good future as a collective of mini-DSP experiences/testimonies and it would be nice if it stays focused instead of becoming diluted with digressions.
@Pogre, you may want to PM one of the moderators about changing the name of the thread to reflect it's evolution to focus on miniDSP