Amp vs Preamp which is more important?

I think you phrased the question well, acknowledging the interdependence of the two. 
 

I have to say the preamp. It takes the really small signal and brings it up to the level the brute can handle it. The preamp is the heart of the system… it brings consistent sound to a system. 
 

Over the last fifty years of my pursuit of the high end, the preamp was generally my most expensive electronic component. I would keep it the longest, then make it the biggest jump in quality.

In general, all my electronic components are in the same price range now (my system is shown), with some variability. In general I would recommend all components would be roughly in the same cost category: preamp, phono stage, DAC, amp, streamer and turntable. But if there were a bias, I would lean towards more investment in the preamp.

If I upgrade soon, it would be my Audio Research Reference 6SE for an Audio Research Reference 10SE (yes, I know that does not exist yet. But the reference 10 does).

Preamp! Tubes! (get optional features, in menu of modern equipment, actual controls on vintage. 

Next, how much power do you need? If not too much, tubes! need plenty, SS.

I need a preamp since I have a few analog sources on my 2 systems. I use 2 preamps that are utterly silent and impart minimal sound to the signal. Both the Holo Serene and Benchmark LA4 are as silent as it gets, and I love them both with every amp I have hooked them up to.

For me, the amp is much more important than the premp. Now if I had a different preamp that added to the sound, such as a CODA 07x, Luxman c900u, or Schitt Freya+, then that would be a totally different answer. 

I firmly believe the preamp is the heart and soul of a system.   Everything goes through it and it drives your amp.    

I had the opportunity to own a Zesto Leto preamp.   It is a seriously good preamp.  It elevated my system's performance like no other component.   I sold it to fund a DHT preamp I am having built to match my amp.      It was a great pre, I took a huge step back using my old preamp til my new one is ready.  You really should try something of that calibre with your amp....

Both amp and preamp can be equally important - way to many scenarios to determine which one. I would think of it as "which component is holding the other one back" and go from there.