Is there a USB driver that you have installed to run with your external DAC?
If so....and if you've not already done so, check this:
With your DAC connected to your player, go into Windows 7 Devices and Printers and find your DAC Device.
If you highlight the device and select properties-settings, there is a list of sample rate settings to select as "supported by the device".
In my case, when I do a clean install and add the USB DAC Driver to Win7, I have to go into this setting and manually select the various sample rates as being supported by the DAC.
..in my case I select them all. (44.1 up to 192) I'm trying to remember if any other settings are necessary... once you are in there to verify the sample rates you'll see other settings which may be of interest.
you can then even do the "test" there, where each sample rate selected will play a bit to verify the operation. (this is without JRiver even loaded)
This is typically all I have to do beyond selecting WASAPI Event and then selecting the actual DAC device in MC.