@FiiO, I've been doing some testing of the coaxial and digital outputs on FW 1.1.5. With the coaxial output I can now play 24/192 kHz tracks but with bit rates over around 5500 Kbps there is an awful hiss over the music that sounds like white noise. Under around 5500 there is no hiss and the 24/192 music plays fine from coaxial.
With USB audio output it has been greatly improved with regard to noise but I am very surprised to be able to control the volume on the X5iii with USB audio output to and external DAC. Also, when I maximize the volume on the X5iii the output isn't as loud as the coaxial output which tells me there is some attenuation or other process before the USB audio output that should not be happening. USB audio output should be bit perfect and unmolested by the source device, in this case the X5iii.
Thought you should know if you're engineers are not already looking I need to this.