In your example, the RU6, being the DAC, is doing the over-sampling in order to execute its digital filters. You also have the no-oversampling option on the RU6, which many, including yours truly, prefer the sound of.
Yep, I use Cayin RU6 NOS with UAPP upsample. The OS mode in RU6 sounds glary/noise with Android UAPP. But I read somewhere it is because android can't deliver the needed power or maybe the processor itself not capable. Or maybe both? Time will tell if I choose M1S, will update here if any difference occur...
Also with Android UAPP it somehow have hidden "gain" when I press to the max volume and keep press it several times and it does affect the power and transient. Not to mention the position of volume in media player, ringtone, bluetooth all have impact on the sound. Thats why I look for transport device than dealing with these hidden variable on Android's UAPP