Hi all. My first post, but I’ve been reading for a while so thanks for the useful info.
I have 1.4 running on a Cubox-i4Pro and initially connected a Musical Fidelity V-Link asynchronous USB to spdif converter (with TAS1020B receiver chip). This ‘worked’ but there were audible glitches during playback. I initially suspected network issues as the Cubox is connected to a small switch and Airport Express acting as wireless bridge back to my Asus router with wired NAS. Playing with buffering etc. didn’t make any difference.
Last night I remembered that I’d built a USB dac using a CM6631A USB to I2S board and separate ES9023 I2S dac board in a small aluminium box. The CM6631A is powered from the USB bus and ES9023 from an external DC supply. This never worked properly with a windows PC - 16/44.1 was fine but anything higher and playback stalled continuously. I’d played with drivers and used two separate win7 PCs so eventually gave up.
I connected my DIY dac to the Cubox and powered up. The CM6631A was recognised instantly and automatically selected by Volumio. I’ve tested wav files up to 24/96 and everything is working flawlessly 
So long story short, if your having trouble with a TAS1020B device you may have more luck with the CM6631A.
Thanks Volumio folk for a great bit of software.