I don't think any browser is capable of playing in exclusive mode to a sound device, so getting bit perfect to JRiver is not possible, or I don't know how!
The following is on a Windows machine:
Make sure on JRiver you have enabled WDM driver.
Add say Superstereo to JRiver streaming list (good channel BTW thanx), play something from superstereo, then on Windows 11, right click on your speaker icon (right hand corner by the clock) and choose volume mixer. Or in Windows 10, right click on speaker icon, choose "open sound devices", scroll down to "App volume device preferences".
Once it opens up, you see different apps and where their sound goes. Default browser for JRiver is "Microsoft Edge webview". Set that to route its output to JRiver.
The music starts to play on JRiver and whatever DAC it is setup to use.You can use DSP and whatever.
I am listening to superstereo as I type.
I have an issue myself!
I can stream from JRiver to my Wiim device (upnp renderer) playing local files, with no issues. But if I use jriver and WDM driver together, it either crashes or won't work.
So far I have not had any replies to my issue from anyone.
Ken