If you are saying that with the Sugden preamp everything worked well when the digital input to the DAC was provided on its RCA input connector, while the input select switch on the DAC was set to select its XLR input connector, that makes little sense.
If that is really what was happening, the only explanations I can think of are that either the input select function of the DAC is malfunctioning, or its design is such that there is some amount of "crosstalk" (unwanted coupling) between its two digital inputs. In either case the condition would presumably be very marginal, such that small differences in grounding conditions that may occur depending on which preamp is connected would mean the difference between the DAC working and not working.
That all seems very farfetched, but those are the only explanations that occur to me if the facts you have reported are accurate.
In any event, if you are inputting to the RCA digital input on the DAC, its XLR/RCA input select switch should be set to RCA.
Regards,
-- Al
