euphonium vs sousaphone

euphonium and sousaphone both are nouns.

Word	Noun	Adjective	Verb	Adverb
euphonium	Yes	No	No	No
sousaphone	Yes	No	No	No

As nouns, sousaphone is a hypernym of euphonium; that is, sousaphone is a word with a broader meaning than euphonium:

euphonium: a bass horn (brass wind instrument) that is the tenor of the tuba family
sousaphone: the lowest brass wind instrument

Other hypernyms of euphonium include bass horn, tuba.

euphonium (noun)	sousaphone (noun)
a bass horn (brass wind instrument) that is the tenor of the tuba family	the lowest brass wind instrument