High end stereo preamps? Worth it?

I just added an Airtight ATC-5 between my DAC and my amp. An obvious difference, more musical, dynamic and relaxing.

an old quad preamp with the spectral tilt control, now THAT was a useful feature!

Many posts saying hi-end preaamps improve sound.
One post saying 'why', surely the additional circuitry cannot improve the signal it is fed with.  Doesn't it just add noise?
Or does it add some aspect to the sound that these posters find pleasing?
Can somebody who understands the tech please tell me.

YES.  When I first plugged in the Audio Research SP-3 pre-amp in 1974, in my shop, I had a true revelation in how systems can sound.

Obviously, in the many years since then pre-amps have evolved and even improved somewhat.  I suggest you work with your local dealer and audition as many as he carries IN YOUR ROOM to find the one YOU like best.

It will be as much a revelation to you as it was to most of the rest of us who have been down this road before.

This is not to say that there are not some SS pre-amps that are very good, because of course there are. However, the one that sounds best IN YOUR ROOM is the one you need to purchase and be delighted with in your system.

Cheers!