What is more important source or preamp?

Source

if its good enough you dont need a Preamp.

The signal from a source through a preamp be it FM broadcast , phono stage or cd, this is where a great front end will hit the wall if the preamp is a bad design at ANY price level.
For anyone serious about this issue not using a preamp, you will need volume controls and certainly not the ones offered with the majority of components.

This is where magnetic based passive volume controls come into play, then again this path may not work in your particular set up ,you have to try one. "However if it does" you can forget the line stage.

Years ago I went that route and it did not work for me,for others they will never return to a preamp.
I will re-visit, try again with these superb controls.

As for a "serious preamp" a good place to start would be Jeffery Jackson's unique tube design, Experience Music Inc. and for solid state Guy Hammels active line stage, Placette Audio.

If in no hurry... wait and see what the new Thor enterprise, sans mr. Marks comes out with... or just snap up an older one for near nothing now. it/they are likely the best value in high end audio today. for their now going prices? A true no brainer... if SE is not an issue for you. Fascinating line stage preamp!

Best preamp I've heard in many many years. Many. Including Krell, Levinson Cary & CJ's upper end ones, but not the very top ones they all make.. as I've not heard their Reference units. I did hear the top McIntosh and again, thor TA 1000 MK II, won easily.

4 tubes.. self biasing.. outboard power sup.. plug it in, play it and you will enjoy it.

The source without a doubt is the most important.
I just corrected my cartridge allignment which was a little off, and that improvement came through loud and clear on all systems.