There is a very good chance that the interference comes from the headphone cable. If you can't detect any interference form 3.5mm, then the analogue circuit is clean. The different between 3.5mm and 4.4mm headphone output is ground connection: 3.5mm headphone output is grounded, so the cable are shielded naturally; 4.4mm headphones cable are not grounded, so the cable become a RF antenna.
Beside, the WiFi and bluetooth module is in the main chassis of N6ii, and the analogue circuit is shielded in aluminum case of Audio Motherboard, so there shouldn't be any internal interference from the WiF/Bluetooth module to the DAC and analogue audio components.
There are numerous test we can do to verify this.
- Replace your Fearless S8F balanced headphone cable with a shielded cable.
- Temporarily add DIY shielding to your S8F balanced headphone cable by wrapping a layer of aluminum foil from the 2.5 or 4.4 headphone connector to the Y-Splitter
- Move the balanced headphone cable away from the N6ii or mobile phone, stretch the headphone cable to a straight line away from these RF devices also help to reduce the level of interference.
Try one of these, if the interference disappeared or reduced significantly, then my theory is sustained.