What qualities stand out in really good solid state preamps?

@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.

@decooney “For a component that is "supposed to be neutral", "adding nothing", looking at history of threads and posts over the past 15-20 years…”

You bring up some good empirical evidence that neutral is not what most people are looking for… although they say they are. 
 

@audiophile1 “I’m trying to refrain from using a term “neutral”. I don’t know if my streamer, DAC, interconnects, cables and components downstream from the source are neutral. There’s just no way for me to possibly know that unless I can A/B what I am hearing at home with what it sounds like in the studio mixed…” 

Good point, but.

There is a way to determine neutral. And it is by becoming intimately familiar with acoustic music in multiple venues. I started my quest a couple decades ago when frustrated that improving one genera would make others sound worse. Over ten years with season tickets to the symphony  (7th row  center) and dozens of acoustical jazz and individual instrument concerts I was quite startled to realize what real music and venues sound like. While there are some characteristics of rock concerts that translates to a system most do not. 
 

When I listen to most systems now they do not sound neutral at all. They are often overly detailed… high lighted details that while interesting makes the venue, miking, mixing, or certain instruments stand out. When cymbals sound like solo instruments, or a triangle grabs your attention then that is not neutral.
 

Bass in natural environments is very nuanced… not slam. My earlier concerns about slam is that the fast slap of bass is not real… in the real world symphonies or rock concerts it is a slower wave with nuanced details. Solid state amps tend to be really good at exceptionally fast rise times and over slap. Now I have realized that even really powerful ones tend to run out of power and therefore do not follow through with the detail.. the articulation of the different frequencies and nuances as the bass arrives. This is something tube amps do well, they reproduce the overall bass experience and nuance well, not pardon the phrase “shooting their load of electrons” on the first wave. Or, at least this is my current theory.

Ok, I could go on and on. But I think neutral is actually the objective of few companies and customers. It is what sounds better to them, which is often hearing things they have not heard before or accentuated instruments or frequencies.   But there are companies and folks that are out to reproduce music as it occurs in the real world.

@ghdprentice correct! If you look at my post listing the qualities I look for in a preamp, you will see that we’re on the same page. Your last paragraph pretty much echos what I stated and sums it up.