I have tried out several options with N6ii (E02) in past few day, some of them are very interesting, so let me share my observation with fellow N6ii users.
(1) The 4.4mm output of E02 is fully connected, meaning the ground is probably in place, so theoretically E02 support single-ended output if you have an 4.4mm to 3.5mm adapter that connect L+, R+ and GND only. I must warn you that L- and R- must not be connected in the adapter, otherwise you'll have short circuit and damaged the E02 and your earphone badly. The adapter will work for both phone out and line out, and I have tried both connections. I am happy to report that both audio output sound "normal" to me, there isn't any audible distortion or background noise. I have consulted our Engineers in advance, he confirmed this works fine technically, but we haven't test the specification of these connection, and he expect some of the parameters such as channel separation and THD+N might be downgraded, but the good news is, E02 can support single-ended line out and phone out if you really needed them.
The adapter below is acquired from a Taobao DIYer. I am not sure if there is similar adapter or adpater cable near you, but if you are handy with soldering, this shouldn't be a difficult project.
(2) One of the strength of E02 is having a clean, "unamplified" Balanced Line Out. You can use your N6ii as streaming/CAS source for your home audio system, or connect to a portable balanced headphone amplifier for mobile application. Cayin is working on a portable balanced headphone amplifier as well but this is only WIP, I don't even have a date for working prototype so I don't expect any exciting news on Q2 2020.
I have identified three portable balanced headphone amplifiers previously, and I am lucky enough to try out the Hum and Romi amplifier at a private meet. I used Empyrean as my reference headphone because it sound very good with N6ii (E02) on its own. Empyrean is the only "portable friendly" full size planar in my collection and I use it with N6ii very frequently. The
Hum MA-1B is very compact and same width with N6ii, so the N6ii+Hum looks very neat and practical. Adding the Hum will open up the soundstage and enhance the control and headroom at demanding passages. The
Romi BX-2 is kind of bulky, but it is very powerful, worth the extra space if you needed that kind of power on-the-go. I only use Low gain when driving the Empyrean. It's driving capability and handling power is very close to a small desktop amplifier, its amazing what you can do with battery powered headphone amplifier nowaday, I definitely looking forward to try out the Romi BX-2 with more demanding headphones, such as my HE6.