So, here is a discussion on the Singer’s Formant oriented middle voice along with some audio examples.
Notice in the first audio example, Bjoerling sings from Che Gelida “e come vivo? Vivo!!” Notice how the chords don’t open when he goes from VI to VO. This modifies the nature of the vowel. The singer’s formant is dominant in that middle voice note. Also notice that the voice is still carrying cavita’, you just don’t hear as much of it.
Caruso then sings from “Ah Si Ben Mio” and you hear how his middle is VERY balanced. There is no excessive exaggeration of the lower harmonics. The voice remains youthful and vibrant.
Then I chose to post a fantastic up close clip of Salvatore Fisichella in concert singing “Bianca al Par di Neve Alpina”. Throughout this clip you hear how the EE function in the chords never stops. All the vowels in the middle are influenced by this closed position, which is never sacrificed.