Is the preamp the “heart” of the system?

Hi George,
You are right... kind of.
When I talk about gain, I actually refer to "meat on the bones".
I lived with passives for decades and when I finally tried an active, I will never go back. I am not a tech guy but I think that this aspect has a lot to do with an active producing musical information that could be referred to as gain.

meat on the bones

Simple, impedance match, just make sure the output impedance is 10 x or more lower than the input impedance, which it is in most cases with passive preamps and today's sources, and in nearly all cases of going to amp direct. This then cannot remove any meat off the bones.

Like Nelson Pass said above above passives.

"It sucks the life out of the music”, is a commonly heard refrain. (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp.”

Cheers George

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