One of the background singers on Lyle Lovett’s “Live In Texas” album has an amazing deep voice. I don’t know who it is, maybe someone that’s already been mentioned. Anybody know?
Since this has been dredged up, I’ll throw in **Barry Carl **of Rockapella fame. Always a lot of fun to listen to.
Incidentally, anyone within range of the BBC iPlayer might want to check out Thomas Quasthoff’s BBC4 programme on Becoming a Lied Singer. Some excellent singing, including a brief clip of Quasthoff performing some solo a cappella jazz.
Since we are raising the dead…
Does Till Lindemann count?
I am shocked that I did not see anyone mention the wonderful William Warfield.
Eastman School of Music grad (B.M. '42). 
As for operatic basses:
Samuel Ramey takes a stab at "Ol’ Man River
Ezio Pinza singing “Non Più Andrai” from Le Nozze di Figaro
Cesare Siepi signing “Fin ch’han dal vino” from Don Giovanni