Here is an interesting discussion of technique from the Bel Canto Society:
What gives [Mario] Del Monaco’s sound its elemental excitement? Ring, ping, what the Italians call “squillo.” Most singers merely have resonance, which in and of itself is never exciting.
Tones without squillo cannot pierce or punch. They may exude sorrow but not heartwrenching suffering or violent rate. For me, singers lacking squillo can never be entirely satisfying as, say, Otello. The full-bodied tones of Carreras and Domingo may please, but they cannot thrill. To thrill, such singers have to rely on the use to which they put their tones, on musical interpretation and vocal acting.
The following have or had voices with resonance but no—or little—squillo: Battistini, Blake, Bonci, Borgioli, Bruson, Caballe, Clement, Corena, Finelli, Freni, McCormack, Olivero, Schipa, Simionato, Tagliavini, Valletti and Vrenios. Though he did not have a great deal of squillo, Gigli, for one, generated excitement through exuberance of manner.
The following have or had squillo: Bergamaschi, Cerquetti, Christoff, De Muro, Escalais, Fleta, Groh, Korjus, Kurz, Lauri Volpi, Martinelli, Nilsson, Pavarotti, Ponselle, Rosvaenge, Ruffo, Tamagno, Tebaldi, Tetrazzini and Zenatello. Sometimes a singer will have squillo on high notes only—Bonisolli, for instance. Sometimes a singer will have it on good days only—myself [Stefan Zucker], for instance. Sometimes a singer will have squillo on the Italian vowels “e” and “I” only—De Lucia, for instance. Sometimes a singer will have it but lose it—Callas and Slezak, for instance. Bjoerling and Caruso each relied on resonance and squillo in about equal proportion.
You can increase squillo by lowering your larynx—but you don’t have to lower your larynx to have it. (Getting your larynx to stay really low while singing or talking takes some doing.)
According to Del Monaco’s autobiography, La mia vita e I miei successi, at the beginning of his career he appeared as Ernesto and Alfredo—and couldn’t be heard. Then he pioneered a lowered-larynx technique taught by Arturo Melocchi, who had learned it in China from a Russian [so this is the Russian secret!]—the technique previously was unknown in Italy. It gave Del Monaco a powerful, brassy sound, but there were tradeoffs: the sound was thick, sometimes muscular, and he had limited ability to color, to modulate between loud and soft and to sing with agility or legato. Often the sound was constricted in the passaggio (the area of the voice where head resonance begins to predominate over chest resonance).
Corelli, to overcome these drawbacks, modified the technique: whereas Del Monaco held his larynx very low at all times, Corelli caused his, in his word, to “float.” Overall the result was more satisfying even if Del Monaco’s B-flats were more trumpet-like than anyone else’s. In any case, the standard repertory sung with a lowered larynx is as anachronistic as Bach played on a concert grand—although the result can be thrilling. (Corelli’s rejoinder is that in today’s theaters, with today’s louder and more brilliant orchestras, singers need the power and steel that come from the lowered larynx.)
… On early records many singers have squillo. Since then, it has become hard to find apart from those who lower their larynxes.
Dr B: Interesting, don't you think? I think there are degrees of lowered larynx, some more extreme than others, but that everyone lowers it at least a little. At least some of the names are familiar.
He says Caballe has no squillo. Well, see her Norma from bel canto society. We need a list that's more up to date.