You have to make sure that you are playing the DD/DTS track and not the MLP (DVD-A) one, which is usually the default playback mode (on all of my discs anyway). This should be selectable from the disc's menu, or try pressing the 'Audio' button on the remote. Make sure you have the video circutry enabled, as on my SD900 it can be switched off to improve audio quality (not sure about the 520?
You also need to make sure the Audio Output is set to Bitstream Out on the Tosh, which it should be if you have been playing DVD's with DTS and DD soundtracks without any problems.
You can also check that your amp/receiver is set to 'Auto Detect' or if it does not have one, or this is not working, try selecting DD or DTS manually on the amp (probably via remote).
May I ask why you are not trying to play back the High Resolution MLP (DVD-A) audio instead of the DD/DTS anyway? Do you not have 6 chan inputs on your amp? You could try the High Res stereo mix (if the disc has one) via the two channel analogue outputs (you may have to select the correct output on the Tosh - Pain in the ass!!).
BTW, I found I prefered the High Res stereo mix of REM's Reveal album over the High Res 6 channel mix.
Good luck and I hope you can get to hear the MLP track rather than the DD/DTS as this is what DVD-A is all about!