This is looking like a Surface Pro issue. There is only one USB port on the Surface so I have to use a powered HUB so that I can plug in the external hard drive with all my music (WD 2 TB) plus the Dragonfly USB DAC. This is the configuration I've been using for months now and have had no problem playing music through MC19, and more recently MC20.
But checking Surface forums today I see that many users are reporting problems when they plug in more than one device to a HUB - with the Surface constantly losing connection with one of the devices. Assume in my case the Surface is periodically losing connection to the DAC (and adjusting the volume re-establishes it?). Some are suggesting this started to happen after one of the recent automatic Windows updates to the Surface. I could understand this sort of issue with an unpowered HUB - but not a powered one.
So basically my Surface is useless for playing my music at the moment. I have more than 1500 albums in FLAC format on my external hard drive - so no option of moving the data onto the Surface internal hard drive
Update: having trawled through all the Surface forums and blogs on this issue it appears this is a widespread problem for which there is no current fix and which Microsoft are studiously ignoring. So my advice is don't even think about using JRiver MC on a Surface Pro - unless you are able to fit all your media on the Surface internal hard drive.