When thinking more about this question: Are the requirements of a Horn of Valhalla cumulative?, I started to wonder if characters could effectively use horns without being able to use their lesser counterparts.
Being able to use Bronze, but not Brass is easily met by druids or many different characters with the Moderately Armored feat. However, I have not found a way to be proficient in all martial weapons without also having proficiency in Medium armor (i.e. to use the Iron horn, but not the Bronze). Is it possible?
I'd like to only include official classes, races, and backgrounds (not including Unearthed Arcana). Feats and multiclassing are acceptable.