A few things, some practical, some preferences.
For practical
1 - On/Off Switch
2 - USB-C is possible (I am trying to unify all my devices to USB-C
3 - Needs to fit in that spot below in the photo
4 - Balanced 4.4mm and 6.3mm out
5 - Bluetooth In
6 - More power, like 50% more power (this isn't THAT important, more future proofing)
Preferences
1 - I like the design, the gun metal look, the red knob
2 - Built-in PSU, no external power brick. This ties into the USB-C idea, any kettle lead can work.
3 - It's a heavy unit, almost a kg so it shouldn't move when i plug in and unplug the cable
4 - Reliability in terms of Fiio. (
https://www.head-fi.org/threads/earbuds-round-up.441400/post-18596993). I was in Head-Fi and someone has a problem with a Fiio product and the company just blanked him. I also googled "Fiio K11 + issues/problems" and found several issues, some little some annoyance, some dead and most relate to the non-R2R version but as a whole, I found some. I then did the same for the XA-02 and I found nothing, not a single complaint. Either it's perfect or nobody reported it or it got resolved quickly. The reliability isn't the whole reason but I realise the R2R side was the reason i wanted to try the K11, and not the K11 itself.