What qualities stand out in really good solid state preamps?

If a listener desires influence from a preamp on the source material (the music as it was mastered), it should be proportionate to influence the kit downstream of it offers. I assume this is the “synergy” perpetually discussed - small to moderate influences of different artifacts in the playback chain syncing up to the preference of one or more listeners. Believable even if not particularly demonstrable. 😉
e.g.: If you want clear-of-artifacts like a Benchmark (preamp) but then run your ZZ Top tracks through a power amp with Shuguang tubes*, you probably keep people guessing in all other life matters, too! 😆

*that’s no dig at Shuguang tubes - in the right rig they’re great fun

In addition to whatever switching you need, and volume control, the preamp's job is to effectively control impedance between your source/DAC and your power amplifier including the cables.   High input impedance and low output impedance does that.  With today's digital sources, preamps in our systems hardly need gain anymore, with the exception of phono sources. 

Most preamps affect the sound one way or another.  Whatever you like about the sound of your system, choose a preamp that enhances the sound and moves it in the direction you enjoy, without screwing it up.  You will know the right one when you hear it.

In my system, I like Steve McCormack's SMc Audio low or no-gain preamps such as the VRE-1 in whatever iteration, which could include the VRE-0.5 or an old TLC-1 that has been upgraded/rebuilt by SMc Audio.   I have tried other SS preamps including Tom Evan's Vibe/Pulse II, Ayre preamps, GamuT D3i, Muse Signature, and the AVC preamps, Acoustic Imagery Jay-Sho and icOn 4Pro.  The Tom Evans did nothing wrong and the AVC preamps were nice sounding and interesting, but the SMc preamp I own is by far my favorite.

@sns - very nice.  I prefer Amtrnas and Audio Note non-magnetic resistors, power supply caps Nichicon superthroughs, Audio Note Caps and V-Caps where I can use them.

I am fine with clicking volume controls. Tells me that something measured -- which I can track -- has happened.

@mitch2 
Thanks for the list of SS preamps you've tried and for the reason you think some are better than others -- viz., ability to effectively control impedance between source/DAC and power amplifier, including the cables.

When you say that the SMc preamp is your favorite, I can't tell why, sonically. How did it sound better?

@benanders 

Good point about the influence from a preamp on source material being proportionate to downstream kit.

@sameyers1 

Why am interested in a solid state preamp?

1. I have three different amps and have recently borrowed a fourth. I like to compare how they sound. Using a SS preamp gives me a tool to compare them without the additives of my tube preamp. Knowing the differences between SS preamps helps me understand which sonic variables might be at work between SS preamps I might put ahead of my amps.

2. I recently compared two SS preamps and heard differences between them. I'm am somewhat uncertain about the vocabulary to use to describe the differences I'm hearing. I'm interested in the lexicography of sonic description, because when I'm reading about A's impression of X preamp or B's impression of Y preamp -- and they're using certain words -- I want some sense of how those words might translate to what I would hear if I tried X or Y preamp.

3. I am trying to establish how big a difference there can be between preamps, overall. Some companies charge 10's or thousands of dollars for their preamps. Why? Could it really be just about noise floor? What makes these preamps so great -- if it's not just runaway marketing and consumer hype?

My present system is here: https://www.audiogon.com/systems/9064

SOURCES:
Intel NUC streamer (DIY) Intel nuc
Holo Audio Spring KTE
PREAMPS
DIY tube preamp 6SN7 (like deHavilland Ultraverve 3)
loaner SS based around Burson design
AMPS
Quicksilver Mono 60s (prefer KT77)
Pass Labs XA-25
loaner Hypex based class D amp
SPEAKERS
Ascend Acoustics Sierra Tower
SUBS
REL R 328
Rythmik Audio (2 subs) F12SE
Cables by Analysis plus.

@hilde45 does your Spring KTE have an optional preamp module installed?

If your goal is to compare amps, it would be interesting to compare the amps by using a DAC’s preamp section. Or you can get Schiit SYS. It’s passive, transparent and cheap. Just a thought.