Maybe this will help. I have an MSB Analog DAC, it can output both DSD64 and DSD128
So to hear either DSD64 or DSD128 files via JRiver to my DAC with no conversion to PCM format, the settings I use are as follows:
"Bitstreaming": Yes (DSD over PCM (DoP))
Which you configure by doing,
Tools->Options->Bitstreaming->Custom->DSD over PCM (DoP) and DoP Format: DoP 1.0
I believe the difference in your case is your DAC will only do DSD64. If you only had DSD64 files you would just use the setting I have above (unless your DAC used the "Legacy" DoP format.
However since your DAC cannot playback DSD128 files, you need to follow one of the two paths Matt indicated for any DSD128 files you play.
Barr