What qualities stand out in really good solid state preamps?

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. 

@hilde45  "2. I recently compared two SS preamps and heard differences between them."

If one of those preamps was more neutral than the other then you may have been hearing the cumulative effects of your source, amp and wires rather than contributions from the preamp.  Two solid state preamps come to mind that others have described (in other forums here and in reviews elsewhere) as neutral or near-neutral and very transparent are the Benchmark LA-4 and the Holo Audio Serene.  These should enable you to compare amps better than your tubed preamp.  I recently removed an excellent solid state preamp from the chain and was amazed by what I heard. Now I am trying to decide if I should keep the preamp out of the chain and only use it when I listen to my phono front end, look for a different, completely neutral preamp, or go for a tubed preamp to ADD certain colorations that are desirable TO ME.  Tough one.

Have used several SS preamps from Krell, Spectral, Mac, Musical Fidelity & vintage Tandberg. I can emphatically say I personally prefer a tube preamp/tube amp setup to compliment my spakers and the rest of my kit however, I recently got to try an Audia Flight Linestage w/phono card that was without question the best SS line/pre I've ever experienced and for all the reasons previously spoken about. The power amp always holds the dynamics & soundscape card but this preamp was A. dead quiet B. is fully ballanced (thats my jam) C. added zero coloring that I could detect 4. visually, looks are stunning and it has the feel of a fine precision instrument. Now, I know little about this company other than what I've now read online after playing around with this thing but I could own one for sure. It would have to play well with my other gear but, looking at the specs and knowing it has an adjustable gain, it would be worth a shot. Maybe next time... just my opinion

• tone - must be natural 
• dynamic range - must be able to sound laid back and gentle but scale up to explosive forces fast, as fast as demanded by the recording 
• low noise floor
• Texture in the bass
• warm and palpable mids
• extended and sweet highs
• nice, non-clicking silent volume control (want to be able to adjust volume without hearing any clicking or other artifacts)
• must image well and create a realistic and colorful soundstage

These are the attributes are what I look for in an amp, except the clicking volume.

I have 2 great preamp volume controls and like the clicking one the best. Though a smooth heavy wheel volume control is luxurious.

This is a very nice list.  For my tastes, I will nitpick and slightly change 

• Texture in the bass
• warm and palpable mids

to just

• Palpable Texture/Mass across the entire frequency spectrum; added warmth I can live without if I get the life like image and presentation.

I settled on the SimAudio 740P 8 years ago or so and that list is what I find it generally gives me compared to a handful of preamps I compared it with at the time and more importantly now when I constantly try different DACs as I swap them in and out of my system.   It's been the one constant in my system over the last 8 years as amp, speakers, and many DACs come and go.   

With so many good DACs having preamp functionality it is so easy now a days to compare what a preamp does well and not so well when you swap in a very good DAC direct to your amp.   I've yet to try a DAC direct (Bricasti M3 and Meitner MA3 are the two best I've tried direct) that beat it in a majority of that list.   I've found the noise floor and ultimate resolution usually slightly win with the DAC but the others generally go to my preamp to my ears.

The other attribute I really like in a preamp is the flexibility and quality of the volume control.   I've tried some preamps both tube and solid state where the range of volume you can make when you slightly turn the volume knob is just too large and I can't get the small changes I want.   I often try to adjust different songs with different recording levels to play at the same volume and it can be difficult if the volume control lacks small increments and doesn't explicitly show you a setting.   

The 740p for instance allows both 1dB down to 0.1dB volume changes depending on the range overall gain level and speed at which you change it, which makes levelling and adjusting to different songs a breeze!