Hi Derek! Yes, "not for the faint hearted." is a rather perfect way of putting it. Years ago, when I was doing club dates and concerts I included a tribute to Mario (of course). I had actually asked Maestro Callinicos to write out the improvised into he did on "Because You're Mine" for Mario at RAH s I was able to include that in my Mario medley. And, I will readily admit that all of the songs were in a lower key. At the time I thought to myself, geez Mario certainly sang these songs in a rather stratospheric key. And you know, he did sing a lot of songs in a higher key (perhaps a good example is "With A Song In My Heart"), but I think that speaks to where his voice lay naturally. Where he felt the strongest vocally. Another interesting point is that he had more vocal energy and stamina when he was a bit heavy. I don't remember if Armando touched on this in his book, but I know Maestro Callinicos told me, on more than one occasion, that Mario's ease with his upper voice was truly remarkable. I think that confidence and vocal security is on full display with his performance of "Lamento di Federico" at Royal Albert Hall. I mean, this is 1958 and Mario's not in the greatest shape physically (or health wise for that matter) and he sings this very difficult aria (I know because I studied it and performed it often. NOT always with the high B - Gigli's contribution to the tenor repertoire apparently). I think though that Mario knew his voice so well, and knew how far he could go with it. Note that I didn't say "push it." This is why I think the RAH recording is so remarkable and precious. Mario is vocally naked out there. And, as Maestro Callinicos noted in his book, the size of both the hall and the audience startled Mario. But his "vocal technique" is on full display. Is it a flawless performance? Not at all. But there are many moments where you can here Mario using his voice so very well. As has been recognized, the RAH recording is a testament to both Mario's talent and his skill as a classical singer.
Sorry to go on Derek. I'm afraid I didn't stick to John Boulter as topic. Just couldn't resist sharing some thoughts on Mario's amazing vocal technique and RAH.
Cheers!