The smoothest amp you've heard?

I comprehend the sentence and premise. Its a very good technical explanation. However, the point I was attempting to make was, from a sonic perspective "smooth" doesn't present itself as something I am attempting to ascertain, or looking for in a amp? I have listened to many amps, I never recall saying "that amp is so smooth". I'm not trying to sound disrespectful. I think its a useless adjective when evaluating sonics of a component. Do you think a sound engineer says "we have to do it again, not smooth enough" IMO, Its a pointless term. 

Some of the First Watt amps , F3 comes to mind.

It doesn't get any SMOOTHER than this:

https://www.youtube.com/watch?v=6Whgn_iE5uc

Adg audion amps are very smooth 

A very nice amp and definitely the smoothest amp I have ever owned was the Luxman M800A. I've owned 36 amps since 2006 (which does not make me any kind of expert, but it does make me psycho!). Certainly the Pass amps I have owned were very smooth as well.
And then I've had some some fingernails on chalkboard amps as well. eewww