The smoothest amp you've heard?

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.  

many tube amps are closed in on top, not airy. so they may be butter smooth, but it’s a circuit artifact overlayed over all the music, and not ’the’ music.

what you want is transparent, grain-less and continuous, but still lively and with great textures, timbres and tonal richness. i’d choose darTZeel; both the dart 108 stereo Mk1 and Mk2, and the dart mono blocks. zero global feedback, minimal parts count in the signal path. purity and transparent. no sense of any artifact overlay.....just music. super smooth and amazingly grain-less and extended, airy top end.

darTZeel does not sound like tubes, or solid state.....it sounds like music.

Until very recently, I have only owned SS amps, and found Accuphase to be "smooth". Whether they lean too much in that direction or not is, I imagine, a matter of personal taste and system synergy (or lack thereof).