Here's the things to look out for when attempting bitperfect playback with external USB DACs:
1. Turn SRC off (as you have done)
2. For DSD, scroll left (swipe finger on screen from left to right) several times to show the leftmost menu, tap Advanced->DSD mode and choose DoP or Native, whichever is supported by your device (usually DoP) to enable direct DSD output to your device.
3. For PCM, if you want to avoid digital volume scaling, tap Advanced->Volume mode, choose Software, turn volume to maximum.
* On some devices you can then turn Volume mode to Hardware and adjust the device's volume directly. On other devices you simply have to adjust the volume using the volume knob / buttons on the device itself.
* Sometimes digital volume scaling is preferable to hardware volume control. E.g. your device has analog knob for volume and your desired volume is very low on the knob's travel range, leading to difficulty fine-tuning the volume and/or channel imbalance. Often the signal-to-noise ratio is limited by downstream analog electronics rather than the digital output bit depth.
I have the Mojo connected to the smartphone at the moment and do not notice any interference noise unless the Mojo and/or headphones (sensitive ones) are placed near the smartphone and/or the length of USB cable near the smartphone. Is this the cause, or is the noise actually dependent on how long the Mojo has been playing from the smartphone as you say?