For a start the loan Grundig is obviously knackered and should not be considered as representative of the make or model.
While you have the loan digibox can you try it on another dish just to confirm that the problem is with the Grundig and it's not the source signal it's recieving from your dish.
There is no way you can accurately rate the various models of digibox based on picture quality, well maybe a magazine could run a bench test but it's still going to be subjective as we all have different eyesight

In a domestic enviroment anyone saying that box A is better than box B only means it looks better on their television. It doesn't mean the the same digiboxes connected to your telly would produce the same result after all cables can make a slight difference and some TV's make a better job of composite or RGB than others not forgetting that digital processing on the TV can make a significant difference, even 100hz scanning can cause problems with digital sources.
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