NI7875 A true bass voice, like the true contralto, is exceedingly rare.
Most basses are bass-baritones, just as most contraltos are mezzos with some extra low notes.
Range, of course, enters into it, but tone sets the defining mark on the category. We can talk of Chaliapin, Pinza, Siepi, Hines, perhaps Talvela, and, in another category, Robeson, of those who had made major careers. Kipnis, Hotter, Morris, Ghiarov, and Christoff strike me more as baritones. As far as operatic work goes, there isn't a lot of call for low notes, particularly when a composer can generate excitement merely by writing few ledger lines above the staff. I first heard Pinza where just about everyone else did - on the original Broadway cast album of South Pacific .