I'm just going from experience here, as I've had both the CTH-8550 integrated amp and the NHB-18NS preamp, and both were dead quiet in their phono stages.
I think a lot of it has to do with the absence of ground loop/hum, since the phono stage is attached physically to the gain stage via a slot, not via some cable. And since, as the saying goes, the best cable is no cable...
But then again, it's just speculation on my part!
Unfortunately, when I had the Soulution preamp here for a listen, it didn't have the optional phono stage built-in, as I'm sure it'd be a great performer, like the line stage itself. I don't know if it's possible to fit the phono after the fact... If it is, that's a great option for current owners of the Soulution preamps!
alexandre