Is the preamp the “heart” of the system?

I wouldn't say so. Speakers, power amps, room, front end, all far exceed in importance. Preamplification is: volume control, selecting input, and maybe making a delicate signal slightly more robust (especially true for phono stage).

Isn't it a an oxymoron to say:
"They  sound finally had meat on the bones and a musicality that I was missing. And it was just as transparent!"  
If it is producing a different sound, more "musicality," then it cannot be equally transparent, which would mean not affecting the music signal in any way. 

I think it's the pancreas of the system.

pancreas/ˈpaŋkrɪəs/Learn to pronouncenoun

1. a large gland behind the stomach which secretes digestive enzymes into the duodenum. Embedded in the pancreas are the islets of Langerhans, which secrete into the blood the hormones insulin and glucagon.

Your all class Wolf

Cheers George