What qualities stand out in really good solid state preamps?

It’s a good point above that whenever considering a pre-amp to consider how well it matches to the amp intended for use with it. Impedance matching is key. A high output impedance combined with a low input impedance will always color the sound and add distortion compared to same pre-amp with proper matched amp of higher input impedance. That’s just a fact! If not considered, one is much better off with an integrated amp where the design takes care of that for you. Tube preamps in particular tend to have high output inpedance and properly should be matched to amp of high input impedance, 10kohm or higher to be safe. Otherwise that clean neutral pre-amp goes to waste. Some might like the results still but you better at that point else the only option is change gear and try randomly again. Not an efficient, predictable  nor cost effective way to achieve desired results.

@sameyers1

I assume you’ve found the solid state preamps you’ve tried are more neutral than your tube preamp, with tighter bass, a lower noise floor and perhaps more extended highs.

That’s on the nose. Exactly right. Often, what the tube preamp adds is preferred. But not always -- some recordings benefit from more the SS preamps additional clarity and upper register forthcomingness -- especially orchestral recordings. I like to hear the strings very distinctly, and sometimes this is not coming through with tubes.

Pass XP-12, eh? I’ll keep that in mind and perhaps be able to get one for a while from TMR or a local audiophile.

@mitch2
Fanstastic list! Thank you for taking the time. I’m saving this!
Surprised Bryston never crossed your path.

@jji666

I’m surprised at the lack of focus on the relationship between the preamp and the amp.

I’m assuming that those who have tried multiple preamps are assuming this as part of the evaluation, already, and don’t need to call attention to it. If not, then a lot of opinions here would likely be nullified if, as you’re suggesting, there may very well be a less-than-ideal match between preamp and amp at work.

@hilde45 Ok, since I’m on a roll and correctly assessed the reasons you’re interested in finding a suitable solid state preamp, I will reiterate my recommendation of the Pass Labs XP-12. While it’s their entry level model I found it better than my entry level Ayre K-5emp. It’s a newer design and about double the cost today of what my Ayre cost years ago. But most importantly it is just slightly on the warmer side of neutral, the Pass Labs house sound, and it images better than my Ayre. If you still want a bit of the warmth of tubes, but in a quieter package with more extended highs and tighter bass I think it would be a great fit for your Pass Labs amp, particularly if that amp is your favorite. If your tube Quicksilver amps are your favorite, a more neutral preamp like the Ayre might be better, emphasis on might, but having not listened to your amp/speaker combo I cannot really say. And of course what I found to my liking might not be at all the same as what you like. 
 

So I’m back to encouraging you to audition several in your system before making a final decision. BTW, Moon Audio has a 30-day return policy on new Pass Labs gear. I suspect there are other dealers with the same policy. Return shipping is usually paid by you on a return. 
 

Hope you find what you’re searching for. 

Beyond a volume control and component switching a preamp can help with matching the impedance between the source components and the power amplifier for optimal signal transfer. I think the reason some folks report improvement with a separate preamp is their DAC preamp or power amp are not compatible. Likely the power amp has too low input impedance relative to the DAC  preamp.

@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.