Is it possible you are attenuating the volume from the source
(laptop) somehow? It seems to me you should be sending a line level
digital output to your DAC from any file source and controlling volume with the
RME, right?
You don't mention how you are connecting your source to the DAC but is it possible the different methods the laptop is transmitting your files are adjusting volume in advance of your DAC?
You don't mention how you are connecting your source to the DAC but is it possible the different methods the laptop is transmitting your files are adjusting volume in advance of your DAC?