First, I'm no expert. Especially on the WDM Driver, which I have not permanently implemented on my HTPC because of the issues you are having, and more specifically, I cannot make the audio lag acceptable when playing video from the internet. i.e. YouTube. Audio lag with respect to video has to be 25ms or less, or it will be detectable, and when using the WDM Driver with YouTube it seems to be in the 100's of milliseconds.
It seems to me that you have read most if not all of the relevant discussions on tweaking the WDM Driver. As far as I know there is no tool to help find an acceptable solution, at least no tool from JRiver. I think one of the discussion talked about using some tools to see the effect of the driver, but I'm not sure.
Anyway, if you are solely using this PC for audio, I would think that making the buffers as large as possible would get rid of the clicks and pops, although it would add a delay to functions like Play, Stop, etc. If you have tried that, then I can't suggest any better buffer settings.
However, have you set the WDM Driver to exclusive mode? If you open the Windows Playback Device, select the 'JRiver Media Center 21" device, open the Properties for the Device, then on the Advanced tab under "Exclusive Mode" select both items, "Allow applications to take exclusive control of this device, and "Give exclusive mode applications priority".
Also on the Advanced tab, under "Default format", you should select the highest bitrate and sample rate your XMOS USB 2.0 DAC supports, or the format that you use most commonly with Tidal etc. Try it both ways. Did you see
this post and the thread it is in? Useful information.
I'm assuming that you did find the
Wiki Article.
Good luck. Keep asking questions. Some others may add more information.