What is more important source or preamp?

Source. If it is not there coming into the pre, it will not be there going out.

Which is more important, a) the source, if it is not clean as clean can be the rest just keeps the garbage flowing, b) amplification, if it isn't made loud enough and cleanly so your going to hear each piece of grunge that gets picked up along the way, c) speakers if they can't release the purity, beauty, and flow everything upstream is wrong.
They are all true, everything effects everything else in the stream from source to speakers to room and back to your ears.
What is most important is getting the stream right and as the I Ching says so clearly, "It is difficult in the Begginning"

No system can be better than its weakest link.

Agreed I have been telling myself get the source right first then the rest will be easy to tell if it's good enough. That's why I worked so hard on the tt and phono stage.

In the early days of the first Cd players,that was the most obvious problem.I didn't buy one until I walked into the record store to find it was all Cd's.That was in 2-4 weeks.I was stunned by what I saw!