Interesting tidbit - it bothers a lot of singers that whenever a Mozart bass sings in an ensemble, he doesn't get to sing the bass line. Rather, he often sings above the baritone parts! In fact, in modern productions it's customary to switch the bass roles' part with one of the baritones, so each voice can sing in their more comfortable register.
A perfect example is in Nozze di Figaro, with Dr. Bartolo. In the first big ensemble number (the Act II finale) the original score has the bass Bartolo singing the entire time in the upper fifth of his range. Meanwhile, the baritone Count sings an octave below, keeping the bassline. In authoritative scores, the parts are written this way, and it gave me a heart attack when I tried to learn the part. But ask a working bass-baritone if he's ever sung that part. Everyone switches the Count and Bartolo's lines. It's so common that in the Schirmer score, they do it for you as an editorial decision.
Why would Mozart, who knew the voice so well, make such a mistake? Is it a problem with the originals? A publisher's messup from the first printing run?
My friend (baritone) Mike Krzankowski enlightened me on the subject yesterday. Apparently the bass who premiered so many of these roles, Bussani, had a lot to do with it. It turns out that he sang these roles as an older singer, when his low range had lost a lot of its lustre. He was much more comfortable in his high voice, and the parts were written accordingly.
And he ruined it for the rest of us.
