My head canon brass instrument for Applejack is the tuba. The reason why is that she's super strong, and lifting up the table that requires two people to lift up by herself is the definite proof. She plays other instruments too, but I know she can't play "everything", but "anything". Anyway, a friend of mine told me that whenever she plays the tuba, its bass voice helps bring out the music by being an anchor, just like playing her bass.