Well, you've gotten plenty of insight. Now I will weigh in as the single-dog owner!
I emphasize that it TRULY depends on your dog's temperment and personality AND that of the 2nd/3rd dog.
Cabeza (our 8 year old Chow/Newf X) has mostly been a single dog since we've had him. There have been times that we had fosters for short periods and even a 2nd dog for awhile. Most recently, we had Mabel, an 11 year old mutt. She was only with us for months before she died of cancer.
In any case, Cabeza seems to be happier alone. He tolerates other dogs, and some of them he will even sorta-kinda play with. But, honestly, when he is the only dog in the house, his energy level is higher, he's more spunky and puppy like. I think that is because he loves US much more. He doesn't like to share and he's not interested in playing with another dog.
So, I would say really really really pay attention to YOUR dog. Don't personify him. What would HE truly like/want? I know that deep down, Cabeza will accept any dog I bring in, but he would really prefer to be the only one in the house.
I want a Newf, and I was always planning on getting her last fall. However, I think I might wait awhile now for a 2nd dog.