Looking for sourced answers, i.e. "Pavarotti says X in interview Y":
When practicing specifically for tone, do great singers listen to themselves
– more through sound traveling though the air to their ears, or - more to sound they hear directly through bone conduction, or - a mixture of the two?
Have singers gone on record saying whether they favor one source of feedback to find/refine their sound?
Both, of course. How would a singer prevent himself from hearing his voice by all available methods? Nowadays they will also listen to recordings.
What is a "great singer"? Opera? Then they consciously maximise projecting their voice which means that the ratio of "bone conductance" to aural feedback will be higher than for other singers. Even then, their larynx should be in a relaxed low suspension (they aren't screeching) meaning that even the bone conductance will be low in relation to the volume reaching the listeners when compared with some other singing styles.
For learning operatic singers it is often surprising just what makes them carry over an orchestra and to the audience since it doesn't feel like that to them. Basically the best projection feels like the sound is not even coming from you but sort of somewhere near the audience.
And what with great pop singers for which the ratio of in-ear monitors (live or else) or headphones (just studio) to "natural" pathways will be non-zero? You'll get yet a different balance for songmakers who match their volume to an accompanying instrument and aim for a more "natural" speech coloring than operatic singers.
