i think it depends on what you mean by the word couple.
if you mean 2 people in a relationship - or merely 2 of something
2 people in a couple, who are a couple as opposed to just two people become singular
but 2 oranges or 2 boats never become a couple in a singular sense, it just means theres 2 boats.
if PS was going to name the two members individually then he would say 'are' but if he was just calling them by their collective name such as 'stardrops' it would be is - though 'the hendersons' would be 'are'.
the hendersons is descriptive though not really a title as such
it would appear though, as lankeea said, that the 'the' seems to change it.
i suppose it depends how you see a band too - as 4 blokes or as one thing - for instance if you know 'the streets' is just one guy you would indeed say 'the streets is number one' - but that is because you are know that he is singular, despite his name sounding like theres more - if you didn't know and thought there were loads of them you would say 'are'.
perhaps as curly says there is no definitive right and wrong and so many factors can alter it.
cor blimey, i know what i mean but i am bu99ered if i can say it...lol