A similar discussion came up on a Denafrips DAC forum. Because the Denafrips are R2R ladder DACs and fully balanced there’s four ladders to support the left (signal A and signal -A) and right (signal A and signal -A) channels. By using a single ended connection only the signal A ladders are contributing to the output being sent to the amplifier.
If you use a unbalanced RCA cable with an XLR connector on the amplifier end I would expect that the signal -A pin on each channel would not be connected. Components that are not fully balanced but do have balanced outputs use something to create the signal -A by inverting the signal. If your amplifier doesn’t have unbalanced inputs then it probably doesn’t have any mechanism to create the signal -A which would mean your amplifier would only be using half of it’s internal bits.
In your shoes, I’d absolutely work towards a balanced preamplifier.