subwoofer vs speaker unit

subwoofer and speaker unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
subwoofer	Yes	No	No	No
speaker unit	Yes	No	No	No

As nouns, speaker unit is a hypernym of subwoofer; that is, speaker unit is a word with a broader meaning than subwoofer:

subwoofer: a loudspeaker that is designed to reproduce very low bass frequencies
speaker unit: electro-acoustic transducer that converts electrical signals into sounds loud enough to be heard at a distance

Other hypernyms of subwoofer include loudspeaker, loudspeaker system, speaker, speaker system.

subwoofer (noun)	speaker unit (noun)
a loudspeaker that is designed to reproduce very low bass frequencies	electro-acoustic transducer that converts electrical signals into sounds loud enough to be heard at a distance