The smoothest amp you've heard?

What's the smoothest amp you've heard? I'm talking something so smooth it was perhaps too smooth, if that makes sense
Ok, if you don't want smooth, go for jagged, rough, harsh, unforgiving. 
Your choice. 

The smoothest amp I have ever had was ARS Emitter, it had a huge power supply and lots of power, it could drive anything and was immune to the mains quality.  It was also the dullest, most boring amp I have ever had.
The best high frequencies I have ever had I have now with my Linear Tube Audio Z40.

It is a David Berning ZOTL design with 6Hz-60kHz (+0 -0.5dB) and no traditional output transformers. So the roll-off (if there is some) entirely depends on what tubes you are using. It is one of the least ’tubey’ sounding valve amps I have ever heard, yet you can instantly tell it must be tubes )). ZOTL doesn’t sound exactly like OTL amps, but matches them in resolution and speed.

The perceived "airyness" is a double-edged sword: often it is just lack of weight in high freq. which is a trademark of so many SS amps, even good ones.

Also, if you go for a "smooth", high-resolution, detailed, "realistic" high freq. you would need to start with your mains supply and vibration control, otherwise you will never get there.

Smoothness is often associated with a liquid, free-flowing midrange/presence region devoid of any edginess. When you start looking for it, you will inevitably end up with tubes. Sone NOS Mullards probably ))