Answering my own question for future readers:
The Wavpack filetype needs to be uncompressed before you can play it back on most audio players.
>From my experience VLC, Audacity, QuodLibet etc all handle the uncompressed files fine (normal 4 channel wav) but have difficulties with the .au compressed version.
The (linux) command to uncompress a file is as follows:
wvunpack
input.ua -o output.wav
Quite simply let wavpack do its job, and play the resulting file in VLC, trough AmbDec to your speakers... :-)
-Harry