I just connected my Diyeden Great March DAC (lower end but decent sound) to my Rowland Capri preamp, having previously connected it via RCAs. The first thing I noticed is that the volume is noticeably lower via the XLRs. I have to turn it up about 5 full clicks to equal the output of the RCAs.
There is an impedance difference with XLR vs RCA. You may have a slight compatability issue. Can you try a dvd player to the DAC via the optical to see if that's the issue? Can you try different inputs in your pre?