The issue so far was, that it was not easy to connect it to recent iOS (ipad, ipod, iphone) devices. They all complained with an error message, that the device is draining too much power after the direct connection between the 2 devices by a regular USB cable.
There are several solutions to it, which all add a lot of weight and inconvenience. One is to convert the digital audio to SPDIF signals (needs an additional adapter which costs 50 bucks) or use a powered USB hub (needs a power source which is not available usually during mobile usage, or a battery needs to be attached).
I think I found an even better solution now.
There is a very compact passive hub which has no housing (just cables, see picture on the right). It is sold by Conrad Electronics e.g. in Germany or Switzerland and probably in other countries as well. It cost 6 EUR only. If the 3 "hubs" are cut away it even looks like a cable. I have not cut them yet on the picture below, I will cut them soon to have a very small and compact adapter.
The adapter has even the right USB connector to the Fiio E17, so there is no addtional cable needed.
To make it work, the USB charging function on the E17 has to be switched off and the hub can be directly connected. Then the music can be enjoyed. See my full setup in the picture below. Very compact, isn't it? With the removal of the non used 3 hub connectors it will behave like a short additional cable (for 6 EUR only).
If you find other sources to purchase the cable globally, please post them in the comments. I was looking for such a solution a long time and found it by accident.
The headphone in the picture below is a Philips Fidelio X1 which I recently purchased. It is one of the best in his price-class. I really like it. To be perfect it is just enhancing a bit too much the bass, but a more perfect headphone (like Sennheiser Sennheiser HD 800 costs 6 times more).