The smoothest amp you've heard?

Maybe you mean the best high frequencies? My nomination is the Futterman H3aa mono OTL tube amps. So rare that most have never owned or heard them! I have a pair - actually my second pair - after stupidly selling the first pair back in 1992. Used with the Quad 57's. 

Smoothest most detailed amp into a pair of Quad 57’s with Kelly Decca Ribbons tweeters from 12khz up, for me would have been a Nelson Pass Pure Class-A 20w project amp I made called the A20, back in the 80’s
https://www.firstwatt.com/pdf/art_classa_20.pdf
This amp led me later to make a self contained recirculating water cooled monster, which was >100w of the same thing, and a two man lift.

Cheers George

All the tube amps I've heard, pretty much all sound very close..If that makes sense.
Speakers make allllll the difference. 
Which is why I am a  WBer cultist. 

Ayre may be that.

It has this weird ability to suck noise out of a room.... you have to hear it for a while to get what I mean.

Luxman L-509X.  Too smooth.