You can do this on MC for Windows (only) in one of two ways. If the input device is ASIO compliant, you could potentially do ASIO line-in which (with some fiddling) will work with just JRiver. Otherwise you need another program to take the USB input and "play" it to the JRiver WDM driver or JRiver's WASAPI Loopback function.
Thank you for the input. It turns out I have a PS Audio NUWave Phono converter which has a phono preamp which outputs as USB. It is ASIO compliant. I loaded the driver and both Windows and JRiver recognize the device. I can select it in Options- Audio but JRiver looks for a library. I don't see how to select a "line in" or direct play function. That function exists in the CD mode by choosing RIP. There is a play box which plays the CD live.
Do you have any suggestions as to how I can configure for live play? My system is very convoluted and complicated. The bottom line is that I have digital, linear phase crossovers on a 4-way multi amped system so everything has to be digitized at some point. The phono cannot remain analog and must be convered at some point. Running it through JRiver gives me considerable EQ and control functions for free. It is all done in the digital domain
prior to conversion to analog.
I originally used this PS box on my Mac to transcribe LP's. I did go through Audacity to make the files and then import them into JRiver. I am now using a Baetis Revolution Media Server which is PC based (windows 10). I understand that Windows does support live play, somehow.