I go by what I've come to call the "Rule of Tens" (although it's pretty similar to an existing precept of "the law of vanishing returns," or something like that). 10 percent of listeners can hear the difference between a $1000 system and a $10,000 one; 10 percent of them can hear the difference between the $10,000 one and a $100,000 one. By doing some fairly simple tests in stores like you've described, I've determined I'm in the second group, hence my own just-under-5-figure expenditure.