What qualities stand out in really good solid state preamps?

Had Pass XP-12 into Rogue ST100 tube amp. No problems and sounded good. 
Matching tube preamps (typically higher output impedance than SS counterparts) to solid state amps (typically lower input impedance than the tube amps) is where you need to be careful. Everything else is a matter of sonic preference. 

@audition__audio 

I think it more advisable to feed your S.S. amp with a tubed preamp and in my experience this seems more common.

It's more advisable if the question is "What's better? Tubed or SS preamp?" But, if you read the OP, you'll see that is not the question. It's not an "either/or" question. It's about the differences between SS preamps. 

I find it very amusing to read that apparently, for some, there isn't audible differences between solid state preamps?!? 

Come on.

Every circuit has a "sound", even when it seems very transparent / neutral, if you compare it with another circuit, you will spot differences. It can be "more drive", "more depth", stronger bass, whatever. Solid state preamps DO sound different, just like the rest.

Many thought that sound exist or the aspects of sound exist as "neutrality" by itself in an absolute way which is all measurable in some objective way for some or separately all evaluated in some subjective way only . But sound perception is an inseparable objective, subjective and intersubjective and historical conditioned dynamic process.Not a mere state.not an isolated factor.

Then the material conditions as difference between two preamplifier be it two tubes one or two S.S. one will be also part of all the material factors at play...

Sound is like perfume blend , it result from an objective recipe determined by acoustic or chemical  and physical materials parameters and subjective impressions and intersubjective impressions but also statistical studies in acoustics or in perfume industry ...

Sound is as sensual and visceral as odor perception in his intimate revelations(human voice timbre meanings) and as abstract and as concrete as visual conditions in their general and universal display (spatial properties of sound and spatial geometry of sound as in echolocalization ).

«The ears are like two noses who had learned together not only to smell the inside but how to see the outside »-- Groucho Marx studies in kitchen acoustic 🤓

«The nose knows in a primal way what the eyes only believes but which the ears can understand»-- Groucho Marx neurology studies 🤓

«The ears think» -- Groucho Marx studies in the womb 🤓

« Then why did the ears do need so much a mouth? »-- Harpo Marx 🎼

«Because silence is already a music in need of a sound»-- Groucho Marx

«Right the dancing body is not enough I need my harp to think and speak »--Harpo Marx 🎼

Music is thinking rythms dreaming a melody.

Any system well embedded and well chosen , with or without preamplification, will give you that.

The only rules are synergetical choices coupled with acoustics...

Each one of us differ by his needs and by his specific constraining conditions.

Marketers want to convince you that a specific  piece of gear is the solution by itself...It is not even wrong but beside the point ...