My take on this (as I don’t have the same situation and devices, it is not based on real experience. Its more educated guesses):
First, the DAC.
If it can be a master DAC than feeding it from a slave device would make sense under the assumption that the DACs implementation has the best masterclock of all.
SPDIF only works if the DAC is in slave mode, so that leaves only USB in the given choices. No real need to do reclocking on USB level as the signal only is a slave signal. But cleaning the signal (on electrical level) can be beneficial to the SQ of the DAC as the DAC receives a signal with minimal polution/interference. Here the USB iPurifier can be a good option.
If the DAC can only be a slave (or does not have a superior masterclock implementation), feeding over SPDIF or USB are both options.
The DigiOne claims to be the best SPDIF transport available taking care of a proper clocking and clean signal. Assuming it is true, and so far reviews claim it is, no need for adding a SPDIF reclocker.
For USB on the otherhand, the Pi(-3) is known for its ‘bad’ USB signal. Both clocking wise and electronically. Here the iPurifier is even a bigger SQ upgrade as it works on both the timing and signal polution.
So, my suggestion: get the USB iPurifier and compare it against the DigiOne and if possible try both Master and Slave mode of the DAC.
As always, just my 2 cents and YMMV.
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