What qualities stand out in really good solid state preamps?

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.

I have kept a list of those I have owned over many years. It probably needs updating but here it is...

Preamps I Have Owned

1. The Best

SMc Audio TLC SE (Signature Edition)

(Formerly a McCormack TLC-1, which was upgraded by SMc Audio and is now a zero-gain, buffered preamplifier with Lundahl input transformers, a Shallco discrete volume control using Audio Note Tantalum resistors, and a choke power supply as used for the SMc VRE-1 preamplifier)(Update 2019: Disconnected volume control and replaced with two Audio Note Silver Tantalum resistors to create a unity-gain buffer for use with a stand-alone, remotely controlled, Khozmo volume control with display)

2. Excellent

Hattor “The Big” Preamp

(Passive/Active remote controlled preamp using Amtrans AMRG discrete resistor volume control/dual-mono with large display/options to operate passively, or actively through NewClassD op-amps with selectable 3/6/9 dB gain, very clear with great tone and dynamics, very close to the best preamps on this list at an amazingly high value to cost ratio)

Lifechanger Audio Smart icOn 4Pro Passive Preamp

(Autoformer, AVC, passive preamp/remote controlled, full function, with awesome display/full-sounding with nice tone and very good dynamics for a passive)

Tom Evans Audio Design Vibe (Lithos 7) with Pulse II Power Supply

(Amazingly real and right-sounding op-amp preamp with “Lithos” regulators, DACT attenuator and an outboard power supply, single-ended only, bare bones operation)

Lamm Industries LL2 Deluxe

(Tubed rectified/tubed output preamp with dual Alps volume controls really nice sound with the absence of balanced operation, remote, and display being the main reasons I still don’t own one)

3. Good

GamuT D3i – good clarity and tone but slightly sterile/lacking body compared to the best

NAT Symmetrical - quite good sounding but a bit tubey in the bass

Acoustic Imagery Jay-Sho * - (Bent Audio Tap-X Autoformer Preamp) Good clarity and tone while slightly lacking drive compared to the best

4. Fair to Good

MUSE Model Three Signature - Owned twice, good body and tone but IMO limited by chip volume control which leads to slightly electronic sound

McCormack/SMc RLD-1 Platinum - Massive drive and good, but not great, tone and body

Atmosphere MP3 (fully decked out with Caddock resistors and Teflon caps) – never grew to like the sound…a little too much pristine clarity for my tastes

Goldpoint Passive – Excellent clarity when used with input impedance of 100K ohms+, but bested by the improved body and drive offered by really good active preamps

Aesthetix Calypso – a little noisy and unrefined compared to the best

VTL TL5.5 – not bad but not special

CAT SL-1 Ultimate MkII

(I see how some would like this but high gain made it sound forced to me)

Sonic Frontiers Line 3 Signature Edition – a behemoth with many tubes

Endler Attenuators

(Passive resistor volume in-line control attaches direct to amps)

Lector Zoe – fun to tube roll since it allows the use of several types of tubes, nice sounding, does no wrong but not as detailed or hard-hitting as others

Ayre K-5xeMP – too clean and a little hi-fi sounding for my tastes

5. Nothing Special

Adcom GFP750 – good for the money

Joule Electra LA-150 MkII – never sounded better than average in my system

Morrison Elad – Ok for secondary systems

BAT VK51SE – thick sounding

H2O Fire – unrefined

Purity Audio Buffer - terrible

* Auditioned in my system but did not purchase

@hilde45 

"Surprised Bryston never crossed your path.''

I always respected Bryston but just never had a good reason to own any of their stuff.  I looked hard at the 28B3 amps for a short time but moved in a different direction.   The thing I always wanted to hear was their speakers, which I suspect may be underrated, but that is just a guess.  Well made gear and a solid company by all accounts.

@soix 

"...everyone needs to define that inherently indefinable neutrality for themselves. I think this is a big part of the art — and wonderful, awful struggle— of ultimately putting together a great-sounding system." 

Great point.  The "struggle" part comes in when people chase some sort of holy grail of sound by constantly "upgrading" to equipment that others report to deliver "neutral" sound, when instead, they should simply trust their own ears and stick with the gear that sounds the most "natural" (i.e., good) to them.