Please what was your issue on M3X and what USB DAC were you using with it?
In its core the digital USB output works the same between our Android devices.
Are you using wi-fi with WPA2 setting? We are aware of that causing problems with download speed.
When I use M3X as USB transport, I get random pop every now and then. I am using Denafrips Ares II. However, with the same set up playing the same music using iPad, the music plays well without any pop.
M3X works well when using it through the 3.5/4.4mm headphone jack output.