Source vs Target: where is the weakness?

In a perfect world mating the best source with the best target you can get is the way to go, but what about the imperfect world. Which would be weaker; if mid-fi source was mated with a very high end target where the source was unable to take full advantage of the target OR a very high end source mated with a mid-fi target where the target might get overwhelmed by the source? Which of the "real world" combination would have the chance of sounding better?

Is it better to have more headroom at the target end or the source end?