Does this mean that its output is not a true balanced design?
No. I took a look at the manual and the specs for your DSPeaker, and the fact that the maximum output voltages are specified as being approximately twice as great for the XLR outputs as for the RCA outputs strongly suggests that the XLR outputs are being provided with a balanced pair of signals.
Given that, and also that you are driving a Pass amp and using a particularly long cable, I would definitely suggest trying a balanced Canare cable, or alternatively a Mogami Gold Studio cable. There is a possibility that the results would not be preferable, depending mainly on the sonic quality of the DSPeaker’s XLR outputs compared to its RCA outputs, but chances are good that there will be an improvement.
Good luck. Regards,
-- Al