I believe I have finally cracked the nut for fine tuning the sound settings when interfacing the (tr)uSDX to a computer using a Digirig. In the video below I’m using Windows, but the same basic concept likely applies when adjusting Alsamixer in Linux.
Bottom line: the Digirig appears to be such a loud, awesome sound card interface for the (tr)uSDX that a massive amount of attenuation is required (-25.4 dB for my setup). Once this attenuation is applied, the transmitted signal actually becomes louder in addition to cleaner!
My recommended Windows control panel sound settings:
Right-click on any other device that is not your sound card interface, and change the “default device” to that other device (like your computer’s built-in speakers and microphone).
Playback
General: change sound card name and icon.
Levels: adjust speaker per this video. Microphone zero dB and muted.
Enhancements: disable all enhancements.
Advanced: no changes.
Spatial sound: no changes
Recording
General: change sound card name and icon.
Listen: continue running on battery power.
Custom: uncheck AGC.
Levels: microphone zero dB and not muted.
Advanced: no changes.