What qualities stand out in really good solid state preamps?

Really comes down to what sound characteristics you’re looking for, but if I was to boil it down to one word it’d be refinement. Specifically, a pre that is naturally detailed and not harsh or etched along with being well balanced and offering good tonality. 3D soundstage is a strength of tubes, so to the extent you can get a good dose of that in a SS pre that’d be a big plus. I think a good example of such things, and given that you were looking at the Herron, something like this Ayre would be in the ballpark…

https://www.audiogon.com/listings/lisb88c9-ayre-acoustics-k-5xe-mp-stereo-preamplifier-56586-solid-state

Hope this helps somewhat, and best of luck in your search.

Is "refinement" a way of saying "neutrality"?

No, not really.  Some SS preamps can sound neutral but but sound sterile and not very musical.  More refined preamps still sound neutral but are more tonally balanced and sound more natural so nothing jumps out at you and you just feel like you’re listening to music rather than some version of it through a stereo system.  I guess that’s the best I can describe it.  The Ayre is a good example of the latter, which is why I mentioned it (plus there’s one available used), and my former Bryston BP6 is another although likely a little less warm sounding than the Ayre — but that’s just an educated guess.

As far as soundstage/imaging, I think in general you need to pony up more $$$ to get to a higher level there than you do with tubes.  @jc4659 is a good example in what he experienced in going from an Ayre 5xe to the considerably more expensive 1xe.  One of the things that attracted me to the BP6, and why I alluded to in your prior thread, is that it produced a large, open 3D soundstage and didn’t break the bank.  Hope this answers your questions.  

@audphile1 Thanks for sharing that pic. That remote is Dope!  Never knew such a thing existed.

I think the problem is with using the term “neutral” as if it’s a fixed and definitive thing when in reality it’s a moving target based on each individual listener. Unless you’ve made a recording yourself, let’s face it, there’s no way for any of us to define what neutral truly is. In that way it’s kind of a waste of time and a fool’s errand to even talk about neutrality because it’s like trying to catch a greased chicken. What I think sounds neutral might sound colored to you and vice versa, and neither of us would be wrong in the context of our own hearing and tastes. And the basic fact is that neither of us knows what the actual performance sounded like or what the final mixed version should sound like because we’re not the recording engineer. Personally, I’d rather focus on what sounds “natural” — natural meaning nothing sticks out and everything comes together and sounds balanced and effortless like real music and, most importantly, just sounds “right.” THAT, to me, is the best “neutral” we can achieve, and 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. But that’s just me.

A number of people who posted comments have dissed the concept of neutral. A simple view of neutral is that a device (preamp, amp, dac, speaker, cables, whatever) does nothing to change the recording. It’s not complicated.

@sameyers1 Oh but it absolutely IS complicated. You don’t know what the original recording sounds like any more than I do. For you to say YOU know what’s neutral is your — and ONLY your — opinion and not based on objective fact at all unless you have the recording engineer sitting in the room with you. Who the hell are you to say to anyone else what’s neutral? It’s just your interpretation based on your ears and taste and nothing more and is not translatable to anyone else. To say that you know what neutral is better than anyone else is just arrogant and silly IMHO.