Per Damian's review:
DTS(MA) in both mkv and Blu-ray structures downmixed to 2ch stereo. TrueHD in a Blu-ray structure downmixed to 2ch stereo. However, TrueHD in an mkv did not get downmixed, but instead was bitstreamed through. This has me thinking that downmix of standalone TrueHD tracks is not supported (in the case of a Blu-ray structure there is an embedded AC3 track which the Dune can use to downmix, no such track exists in a TrueHD mkv).
For HD Audio over HDMI, the TV303D does not downmix to multi channel PCM natively, only 2ch stereo. Using the custom patch you can add back multi channel decoding of DTS(MA), but for TrueHD you still only get the original TrueHD track bitstreamed (as mentioned above)
XBMC has a much better looking interface IMO but I got to the point that I just wanted a player that worked well and was reliable. Zapptiti looks good enough to me, is free and serves my needs.
XBMC running on the Dune Max will be something I'll take a good hard look at.