Back to the beginning of this thread, Charles made the claim that the preamp is the Heart of the System. Dave disagreed stating that the amp was the "heart".
I don't claim the same degree of knowledge of either of these gents, but my experience is that a preamp brings out whatever the amp has to deliver... so as far as I'm concerned, your system will only sound as good as both your preamp and amp. But if I had to pick, I'd still say that the preamp is the key piece to my system.
I would say amp, too. Once gain is involved everything changes.
You can have preamps that impart a lot of character, too. And then pair them with an ultra low distortion amp (trying to be invisible), to sorta have the preamp be the heart. But I prefer a preamp that can drive amps better, so you get some improvement but it's really invisible and not lacking an ability to drive the amp. Then the amp has character and is the heart of what you want.
A preamp that's just a volume knob can't do anything to improve sound by driving an amp better. It would be pretty hard to call that a heart.
Here's an interesting example. Mola Mola. The preamp has got accolades because it doesn't impart a bunch of sonic character, just drives amps better. But the Mola Mola amps are not really cared for at all because they're lifeless in many people's opinion.