What qualities stand out in really good solid state preamps?

Recently I posted on the Herron HL-1, asking people what they thought, how it compared, etc. It's been sold and that's ok. The search continues. 

But it raises a question I'd like to ask folks:

What attributes do you look for in a good solid state preamp?

Some qualities — quietness, durability, seem pretty obvious.

But what other criteria do you use to differentiate between solid state preamps?

How can they differ and what matters to you?

Please let me know!

P.S. As I've looked around, I've begun to learn more about some of the legendary preamps — made by companies such as Threshold, Ayre, Bryston, Pass, Apt-Holman, and others. It's good to have these names as references, but it would be even more useful if I knew what these brands conveyed, sonically. I've played with the idea of getting a newer Schiit preamp and then I wonder -- what if there's a "classic" preamp out there, used? What would it deliver that was worth searching for?