When you add a 16x digital filter board to an older Platinum DAC, you need to make the following upgrade to get the best performance.
This upgrade is required for motherboards of REV 5.9 or lower. Any motherboard of REV 6.0 or greater does not require the upgrade.
Remove the cover and remove the DAC modules.
Scratch the green solder mask from the traces at the places shown.
Tin and solder 220 ohm resistors from the trace to the ground plane. This is done in 6 places.