VLC Mac AC3 downmix optical out to 2ch DAC voice too loud

Hi,

I have some BBC HD mkv files of the program "Later...with Jools Holland" encoded with 5.1 AC3 sound.

I have optical out to a 2ch DAC. Audio MIDI set to 24bit/48kHz over optical out.

Playback VLC: vocals very forward and loud in the mix, instruments in background and too soft. I also have the 2 channel SD version of the broadcasts and the mix is perfect.

Any suggestions?

Thanks, Ben