The smoothest amp you've heard?

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 ))

Luxman m900u.  Smooth, detailed, airy and you can listen for hours.  Will be updated early next year with the m-10x if the rumors are true.

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.

@jakesnak  Its not. In fact, if the amp isn't sounding nice and smooth, to me its an indication of higher ordered harmonics and I will be looking for a reason to work on the circuit 'till I have it right. It sounds to me like you've not experienced an amp that is actually smooth. My recommendation is to listen to the difference between a solid state amp and a tube amp (one in good condition of good design; not something hauled from a 1960s console...). If you've only heard solid state, you might have heard hundreds of them and not heard anything to justify the term. There are smooth sounding solid state amps but they IMO/IME are quite rare.


One qualifier: When/if you are interested enough to do this as an audition, make sure both amps are happy with the loudspeaker used. If that is the case, you won't have any problem hearing this difference- its literally why tube amps are still around after tubes were declared 'obsolete' back in the 1960s. Normally when the prior art is succeeded, it goes away. But tubes are still in production; they have been 'obsolete' now for longer than they were the only game in town! That's happening for a reason and that reason is literally 'smoothness'.