You've Lost that Lovin' Feelin' (1964) certainly qualifies as an early one.
Another was Roy Orbison's "Oh, Pretty Woman" (1964)
Actually, 1964 was a pretty good year for rock ballads. "House of the Rising Sun" was another:
It's pretty tough to beat a 1964 rock ballad played by guys in matching brown mod suits. If this world was a reasonable place, which of course it will never be again, rock bands would still be wearing matching suits, mod or otherwise.
I will never forget seeing my first rock show in 1962, featuring a band with guys in matching cranberry colored greaser suits, slicked back hair, and sh!t-kicker shoes pointy enough to cause significant damage, with side-tie laces. I was a kid. But at that moment I knew I wanted to play in a band like that. Good times. Of course, by the time I was in bands, no one wore matching suits for gigs any more. I consider this one of the great mis-timings of my life.
"What were the other woeful mis-timings like?"
"Well, one was being a passenger on the Titanic. Stuff like that."
How much bass a song was mixed with is entirely irrelevant, in my book. But if you need a lot of that guitarry stuff to float your boat, I'd submit Zep's "Dazed and Confused." (1969)