In many ways it's an impossible question to answer as for many people the last thing they want is for it to be good.
A perfect pre amp has no sound of its own. Most who want to add a valve pre to a SS power want to ruin the sound by adding "nice" distortion and in many ways and many circumstances having a bad "match" will "help" this!!
I went from Audiolab 8300MB and Farlowe era pre', changed to Avondale NCC220, changed to Croft Micro, changed to Croft Super Micro.
Each change brought greater clarity, greater precision, greater depth to the music, and the changes were easy to hear - I have an unbiased guinea-pig to judge.
Did the valves bring distortion? To be honest, the changes for the better swamped anything else that my ears may tell/told me, and what the speakers produce, the ear collects and sends to the brain, and what the brain interprets that as, are all, very, very different.
Yes.... well the lack of NFB and no doubt excessive loading of the valve stage did. More than that of the other gear anyway.
As I said above, and will keep saying, the best pre amp is one with no sound of its own... "a straight wire with (or without!) gain". This is the raison d'etre of hi fi itself. Reality. Not "niceness".
Serious request - please explain a bit - I don't think I read that comment as you wrote it and meant it to read. "Excessive loading" in particular.
I use a valve preamp to drive my diy Voyagers via eight metre leads. Sounds absolutely fine to me. I use another similar valve preamp to drive a Crown PSA2 or NCC200s in my office system. I've tried a few SS preamps and not liked any of them. I tell a lie, I did quite like the old MF Pre3a I had but it was a touch noisy.