Among the great tenors of yesteryear, the heldentenor to beat them all was Lauritz Melchior, also called the Great Dane. The quality of the recordings are not up to today's standards, but the voice! What a voice! And his singing was always effortless. He sang at the Metropolitan Opera into his 70s!
For a real dark voice like Jonathan's try Ramon Vinay. The Canadian Jon Vickers is another of my faves along with American Jess Thomas. These were great singers of the 1940s, 50s, 60s and 70s.
Nowadays there are few great ones. Two of my favorites are South African Johan Botha and German Jonas Kaufmann. Neither has a voice as dark as Jonathan's, but their techniques and range of color and dynamics are superb!
Melchior and Flagstad - the Tristan & Isolde duet. It’s on YouTube and the sound quality’s not bad at all.
Thanks for the great recommendations. I think I’ll first try to find a Melchior LP or two. (Have become obsessed with the qualities of record-player sound since getting one for Christmas.)