Hey Audio Bug,
its a constant problem - what DSTV say and what they actually do, many of us have come to know are two very very very different things. They promise 5.1, its 2 channel, they promise HD, its actually upconverted SD...
same old story over and over again.
There are countless threads over on the DSTV forum if you are interested. Most of the time they seem to get at least something right. But then there is always the annoying adverts louder than content stuff, or the wrong aspect ratios, or the loss of signal for no apparent reason etc. lol... the list is endless...
anyways, there is nothing you can do about it. your amp probably works absolutely fine. Try the language button sometimes - it helps switching from surround to stereo when the 5.1 soundtracks are particularly bad - eg lipsync, or distortion... or in some cases, the completely wrong audio.
When it is on 5.1 though you should see the DD logo light up, and play in full 5.1 (or sometimes 5.0.... argh, again!) and then when they broadcast in PL2 then your amp should still give you surround, but it will matrix it out from the pl2 signal from dstv's broadcast.
hope that sheds some light....
my 2c
cheers,