I just uninstalled the topping driver to use the Windows standard driver.
Now the popping after no sound is played has disappeared or only occurs very rarely.
I feel it works better with the topping driver. If you try the Speaker Properties test under the Advanced tab (where it plays a test noise through the L and then the R channel), with the Topping driver it pops a little on the first sound and then 3 seconds after the test finishes. With the windows driver it pops on the sound from both channels and is delayed too. I think this was mentioned earlier in the thread somewhere but if you max out the Windows volume then you don't get the pops, even though the sample rate still updates on the display.
I'm no expert but it doesn't seem to me like a hardware fault (specific to certain units), it feels more like a characteristic of the design (not necessarily a fault per se) that could be addressed (or worked around) through either a driver or firmware update.
That said it would be interesting to know what causes the DSD64 issue on some (but not all units). Maybe a tolerance thing.