Do I need a subwoofer for music, or only movies?

Music benefits too — most bookshelf speakers roll off between 45 and 60Hz, and kick drums, bass guitar, and synths live below that. The difference: movies demand output (effects at high SPL), music demands integration (a sub you cannot locate by ear). A properly crossed sealed sub at 80Hz improves a music system more than most speaker upgrades at the same price. If your speakers are towers with real sub-40Hz extension in a small room, you can defer it.