I bought a 2nd hand so called Hong Kong Lawrence CS4397 DAC a few years ago. The original owner modded this DAC and I am happy with the sound.
Pictures of this DIY DAC:
I also have a Raspberry Pi and I was struggling how to setup a high quality audio system based on Volumio and the components I already have. This means that I need a USB to SPDIF converter (maybe this one or this one) or I2S to SPDIF converter (maybe HiFiBerry-digi) in case I want to use the Raspberry Pi. Or another option would be to move to the UDOO with its onboard coaxial SPDIF output.
What would be the best solution in terms of audio quality?