Is the preamp the “heart” of the system?

YES

Yes, yes... a straight wire WITH GAIN!
Passives mostly have the straight wire part but not the latter.
But “uncolored” is highly subjective. I think that preamp manufacturers all have their own idea of what music sounds like and embues their pre with their “house sound”. Or “no sound” if you will.
But no sound is not necessarily desired. I think the safest and, hopefully, the best choice is to match the preamp with the amp by using units of the same manufacturer. I have found an amp in which I love the house sound, Atma-Sphere. I believe it is unique and special. So, I want to emphasize that sound by using an Atma-Sphere preamp. I think and hope that this match will be my destination units. And the amps work great with my speakers. Off the merry-go-round!

mglik OP
Yes, yes... a straight wire WITH GAIN!
Passives mostly have the straight wire part but not the latter.

Sorry but where have you been, you don’t need any more gain, as 99% of sources today have enough voltage output to clip any amp to full output, you don’t need even more from a preamp.
The reason preamps had all that gain was from the dinosaur era when they had phono inputs, phono cartridges need it.
There’s no need for any gain today, even with vinyl, as today’s separate phono stages have 60db even 80db gain, even they can go direct with a passive pre as the volume.

Nelson Pass

“We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.

Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.

Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.

What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.

And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp.”

Cheers George

Hi George,
You are right... kind of.
When I talk about gain, I actually refer to "meat on the bones".
I lived with passives for decades and when I finally tried an active, I will never go back. I am not a tech guy but I think that this aspect has a lot to do with an active producing musical information that could be referred to as gain.

meat on the bones

Simple, impedance match, just make sure the output impedance is 10 x or more lower than the input impedance, which it is in most cases with passive preamps and today's sources, and in nearly all cases of going to amp direct. This then cannot remove any meat off the bones.

Like Nelson Pass said above above passives.

"It sucks the life out of the music”, is a commonly heard refrain. (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp.”

Cheers George