Further to my earlier query (post #4103 above), I have done some more playing today and it would appear that CyanogenMod breaks the native support for USB audio on the Galaxy S3
Trying some other ROMs appeared easier than reverting to stock so I've ended up with OmegaRom v56 (Android 4.3 based) which works perfectly in all regards, although it appears to be just as 'bloaty' as the original Samsung.
I now have a very nice DAP based on the S3 and FiiO E18 using USBAPP for lossless files on a 64GB SD card, Spotify 'extreme' quality for streaming and downloads, Soundcloud/Soundcloud downloader, and TuneIn Radio Pro
Given the availability of products like the E18 and the reasonable second-hand prices for 'high-end' Android handsets, there's a real opportunity for a minimum function set custom ROM / Kernel that exploits the best audio performance and maximises battery life. Hopefully some of the clever developer community will be inspired to produce one.