How important is the pre-amp?

Used integrated amps for ages, till i got my firstwatt SIT-3.
At first connected to dac direct, rockna wavedream dac has some digital volume control.
Loved the amp enough to decide to get a pass XP-22.
The preamp arrived and i got mind blown!
There is "drive" even at lower volumes.
Sound stage expanded in all directions and everything within it separated beautifully.
More details which is counter intuitive! Supposed to be the opposite, but it was not.
Even tone and timber improved.
I highly recommend a decent preamp. Its not just a volume control, I'm a believer now

The preamp is very important. I just purchased a new preamp and it has completely changed my listening experience.  I thought I had a bad match between my speakers and amplier but the problem was the preamp. I found this out my accident. I only purchased the preamp because I was on a buying frenzy. I wasn't expecting a change in the sound signature at at all. If you are not happy with the sound of your speakers try a new preamp.

I don't think the preamp is the heart of the system, but I'd go with pancreas for sure. 

Interesting discussion. One topic I haven't seen mentioned is how the "Line Level" of a DAC or preamp output affects the equation. When I was young, I was taught that the main purpose of a preamp, assuming you don't want to color the sound, is to receive an input signal and output an uncolored signal to the power amp at "Line Level". So, in theory, if you have a good DAC (with a good volume control and analog output section) which provides an analog output at the same Line Level as a given preamp, how could the preamp do anything BUT add coloration? But, if you look up Line Level on wikipedia (https://en.wikipedia.org/wiki/Line_level), you'll see it's not quite that simple.

To use a specific example I'm familiar with, the Gustard x26-Pro is reputed to be a pretty good DAC, and some of the posts on this site have said that it's analog output section is a good one. Yet others will say that feeding its output directly into a high quality amp provides noticeably poorer sound than using it with a good, uncolored SS preamp, such as a Benchmark HPA4. Why would that be? I don't know the answer, but I'd bet it has something to do with the DAC's analog output being unable to match the Line Level of the output of the HPA4. This doesn't make sense to me if the DAC truly does have a good output section, but I trust the people who tell me it matters to them. If anyone would like to weigh in on this, I'd love to learn more.

I took out my Ayre KX-5 Twenty a few times to test if the cost is justified. I immediately put it back in with a dCS Bartok & Lumin T2 (demo) going to an Ayre VX-5 Twenty. My experiment to omit the preamp backfired as I ended up buying an Ayre KX-R Twenty once I experienced the importance of a good preamp. It doesn't matter that the Bartok has a 6v balanced output. I wouldn't consider running raw dog unless I had a MSB Select II like @mikelavigne.