the smoothest sounding speaker

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.

and the kit ZRT

Low distortion
Wide even dispersion
Flat frequecy response
Well Damped and Excellent transient response ( & no dull compression)

The speaker that I have found to strike the right balance between resolution and smoothness is the Merlin VSM-MXe. The other speaker that seems to be as good are the various speakers I have heard from Verity; if I didn't own Merlin, that would be the speaker line that would most interest me. I also owned the Vandersteen 3As/2wqs, but the Merlin was an improvement in the area resolution and micro dymamic detail. The Merlin uses the Dynaudio Esotar tweeter and I imagine that is a reason for its smooth upper frequencies. Of course, using tube amps doesn't hurt.

For example here is a distortion plot and cummulative spectral decay of a mid range driver. Nice because it is playing at 96 db SPL at 1 meter (that is real world music loud - so it is playing at useful levels not soft levels for the test - this takes care of dynamics cleanly).

Distortion and Cummulative Spectral Decay (CSD) Note the CSD goes to -35 db which is an entire 10 db lower than you usually see on Stereophile plots - and remember you can ignore the breakup/resonances above 4K as this would be removed or taken care of by a crossover (it is a mid range driver - it can do tweeter duties)

The dispersion is nice and even also - see Dispersion plot