Glenn has also said that during the recording of Burn (I believe) he sought out advice from Stevie and Stevie said something to the effect that Glenn should hold back a bit (or something to that effect). As a singer myself I take that to mean that perhaps Glenn was pushing or trying too hard, as in either pushing his voice hard (he was singing rock after all) or maybe he meant that Glenn had a little too much tension in his throat when singing soulfully. I can her this some in the ballads on the "We are the Music..." album. Just a guess though as Glenn didn't go into details. Still, I like his singing from that period. "We are the Music.." is Glenn's vocal break out album, IMO and you can hear him experimenting with his very gifted voice quite a bit. Noticed he settled down on Burn - much more disciplined, controlled and relaxed.
P.S. I'm glad DP gave David Coverdale to the world (a great baritone), and those glorious harmonies too, but I would have loved to have heard what Glenn could have done on his own. Gillan who? (No offense, love him too).