the smoothest sounding speaker

for sheer smoothness, I wld think Ascendo Systems line of speakers would be right up there

You have many options, but two speakers that are remarkable for their smoothness are Sonus Faber Cremona, and Zu Druid. The Cremona is a voiced speaker that trades some ultimate resolution for musicality, and the Druid is an intimately holistic reproducer that intentionally sacrifices some of its big brother's extreme revelatory character to retain the warmth and unity of a simpler design. Two different price points, yes, but both can be used with a wide range of gear, including the best.

Phil

in striving to build smooth, detailed speakers i've found that the source is sooooo important. much of the harshness and unnatural sound that speakers get the blame for is often the source component. really good speakers reveal the weakness’ and strengths of the other components. often it's really hard to tell what is the component that is the 'problem'. right now i use an Art Audio tube DAC for listening evaluations.

I hate to argue with you Lou but in my experience it is the loudspeaker that is most often to blame for fatiguing sound. Before I found and fell in love with your DA-1's, I heard countless speakers--most often connected to top shelf electronics (both tube and solid state)--and often found them to wear on me after a relatively short period of time. There are very few speakers out there that get things so right that you can listen (at volume) for hours on end and never feel oppressed. I'm not saying that poor electronics can't mess up an otherwise good speaker but for the most part I think you need to get the loudspeaker right first and foremost. I think your success in this area transcends choice of electronics and source components.

Here's one When the drivers like this have incredibly low distortion, and the design is well implemented, you get a non-fatiguing "listen all day" type of sound

I think (as of today) driver distortion is my biggest detterent to enjoying the music. I can live without all the detail, bass, or treble, but if distortion is high.....no way.