I am using the fireface for output yes. This used to work fine with my old device (focusrite saffire pro 24).
As I said, some folks have gotten it working with some devices, but I've never personally gotten it to work and the dialog box warns that it might not work. Someone else with a Fireface may have some insight.
How would I use JRiver as the input here?
The use case is primarily measurements but some use of redirecting YouTube etc through JRiver. I don't do much of that though.
JRiver has an ASIO driver for input built in as of version 19, so you should be able to select JRiver's ASIO driver as an output option in any ASIO capable software (i.e. most measurement software). JRiver shows up in the list of available devices, and choosing JRiver as on output option delivers the output of the external software to the ASIO line in, which is then routed through JRiver, etc. Some measurement suites insist on using the same device for input and output, so that obviously wouldnt work in that case.
As for youtube, web-browsers aren't typically ASIO-capable, so for redirecting youtube I can recommend using WASAPI loopback as an alternative if you can't get the JRiver/Fireface nexus to handle both input and output.
EDIT: Actually, I just realized we discussed this once before, and you had some trouble getting the ASIO driver going:
http://yabb.JRiver.com/interact/index.php?topic=88927.0 If you're game to try and troubleshoot I'm happy to help.