Denia will be fairly quiet in September as it's a typically 'seaside resorts', and bars, shops etc may be closing more towards the end of September.
Gandia is lovely, more typically for Spanish tourists and again will be quite as the Spanish will all be heading back to Valencia now doubt. More Spanish spoken here than English. No English Breakfast bars etc.
Not been to Calpe, so can't comment, not heard great things, but everyone likes something different.
If it were me probably Javea, Moraira and Denia area.