I have never used a sub before but in respect of finding the cut off frequency that would be possible by the adjustments on the sub and using your ears..
I don't think this is the case with the D03 as the cut off frequency is defined in software and the sub will only provide the set of frequencies provided from the Sub out RCA connection. My understanding is this output is not the full frequency spectrum just the lower frequencies specifically for the Sub. So setting the controls on the Sub will need to take this into account.