Why do you care? it’s just vinyl.
70db max dynamic range
My understanding is that the resolution of the ADC will be the limiting resolution overall for this type of unit. And you only have the maximum bit depth to begin if you input the analog signal at or near the highest possible level for the ADC -- not a good thing, apparently. In Amir's reviews of ADCs, he sometimes finds it necessary to back off the rated input level to avoid generating lots of distortion.
Once in the digital domain, you will then throw away bits from whatever you had to start -- with EQ and any other DSP, as well as the digital volume control if there is one. (I don't know whether the miniDSP has a fixed analog output.)
So if you take 70 dB as the maximum dynamic range you might need for LP playback or an RTR deck, then -- so I think -- you need to allow some unspecified further margin. Is 20 dB or so not a reasonable margin?
More generally, it seems better to have more resolution than one will actually use -- sort of like 24-bit DACs being a desirable thing. I'm open to new information, and would be much obliged to anyone who can help me see how the above isn't the best way to think about it.