What qualities stand out in really good solid state preamps?

Great post!

Especially true if we refer to gear pieces to define and learn about any concept instead of acoustics as a reference, the way we create ourself "neutrality" in a system room teach much more than using a new piece of gear...

Then for reason relating to the characteristics of our system parts /room and specific synergy what we look for will differ for each of us. For example i was looking for a more "organic" synergy , then i pick up tubes preamplification to couple it with my dryer active speakers.... Luckily it was a match in heaven....But the same preamplifier may appear as "trash" for another person needs, and system/room characteristics and level ....There is no rule  in brand name choices preamplification  for a synergy... Electrical specs compatibility but it is not enought to create synergy which ultimately is an acoustic experience...

Growing up where only English is taught/spoken and moving to/working where several languages are taught/spoken (and where English is a second language), it became clear the inevitable pitfall of trying to tie down meanings among people for audio kit adjectives that have been adapted from other senses (at the most proximal) or completely unrelated contexts. This seems a very fast-moving target be it for preamps or something else.

But admittedly, the notion of potentially feeding a SS preamp to a tube power amp had me curious just for how unconventional it is!

They are typically very quiet. Neutrality but not at the expense of texture and proper color. Most S.S. preamps I have heard dont get as many things right as the more neutral/linear leaning tubed models. Perhaps oversimplification would be a good word to apply to many S.S. preamps.

I think it more advisable to feed your S.S. amp with a tubed preamp and in my experience this seems more common. Very few if any disadvantages to tubed small signal devices versus tube amps which need to be matched more closely with speakers. Especially true of my OTLs. 

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.