Amp vs Preamp which is more important?

You have an $18K amp paired with a $2K preamp.  While I've not owned either product, I would bet there's a good likelihood that the entry level preamp is holding back your high end amp.  I'd only be looking to change out the preamp, unless the amp is not a good fit for your speakers.

The amp is far more important. Why? Because there are many components that one can use with the amp that have preamp capability and functions. Many will keep the signal path simpler and negate the need for a preamp.

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.