The only singer I've heard approach Tagliavini's sweetness and softness(at times) is Gigli. Here is one where they are both equal, or at least, so good that it's difficult to decide between them. This is one of my favourite songs of all time. I love Tosti. And yes, I have and adore the Schipa version of this, but he doesn't really fit the current contest.
Tagliavini
https://www.youtube.com/watch?v=tQCX0i2mC3Q
Gigli
https://www.youtube.com/watch?v=xXbKaZ4vI0k
Since I rarely get to do this, as their paths don't often cross, here is Carlo Buti, known for his sweetness.