From the sideline, it doesn't appear that you've done enough troubleshooting yourself.
From this info:
http://nadelectronics.com/products/hifi-amplifiers/C-390DD-Direct-Digital-Powered-DAC-AmplifierThe NAD C-390DD comes with USB ports out of the box that give the following performance:
USB Type A - 24 bit/48 kHz
USB Type B - 24 bit/96 kHz
Note that you need to load relevant USB driver software to your PC to get these ports to function (available from the download section in the noted link).
How about you see if the USB digital out works?
With this style of amp, HDMI is enabled via a plug in module. How do you know that the module itself or the amp isn't faulty?
Before you start blaming the software, you need to show that the other relevant possibilities are not an issue. In fact, from what you've described it would appear that MC is producing digital sound within the operating system. This tells me that the MC software is probably not the issue and it's quite possible that you've got a hardware problem somewhere in the mix, if configuration of the Sounds function within Control Panel doesn't help. It might be ports on your PC or ports associated with your amp.
Can you try loading MC to a completely independent PC and isolate the problem that way?
I sympathise with your frustration but I don't think you're seeing the true value of the forum. If you engage positively, seek out your issue by searching and persist, you will get a return on your investment. I've had a generally positive success rate when I've had a problem, so I'm satisfied that the forum can function and does help.
And to be a little blunt, this tech support delivery model is the way many outfits work these days. Many big name tech brands use this methodology (I can think of Toshiba, Sony and Asus from recent experience off the top of my head). Even Microsoft's so called help function (good ol' F1) in its Office software will often kick you out into the forum space on the internet and the days of a paper manual are long gone. One reason for this trend is commercial reality. Forums have organically developed as a way of reducing tech support overheads. Given their proliferation, I'd say they work for the developers and that then flows on to users in terms of reduced costs.
So, unfortunately if your tech support expectations are stuck in the past, I can't see that your frustrations will ease in the near future. My apologies for bringing this up here but you're not the only person on the forum who has pointed out the shortcomings of the tech support model employed by JRiver. I'm purely a user and I think it helps the development staff sometimes to have other voices come along to gently but directly explain the way things are.
Just my opinion and we may agree to disagree.
Hope you resolve your issue soon..