I've been to Dargaville. Once. It never occurred to me that there was a reason to go back. There is also a good display of Maori musical instruments at the War Memorial Museum in Auckland.

I live near Edinburgh, which has the Reid Museum, with a great clarinet collection.

In London there is the Horniman Museum (like the Reid but worse displayed).

The best I know of is the Museu de Musica in Barcelona.