the smoothest sounding speaker

How about Ohm Walsh micro talls or Eggleston?

Spendor S8e. Very smooth with wonderful midrange texture, strong tight bass, and beautiful non-etched relaxed and nicely detailed highs. Their fuller bass and midrange texture won my vote over some Dynaudios.

10-16-09: Cdc
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.

Then, I would think electrostats or planars would be your first, and natural, choice.

I agree with Pubul57 choice of the merlins and would add Dunlavy scIII,scIV,scV and scVI's into the mix;these are speakers that you could listen to forever.
I would also vote for my current speaker soundlab m2's except impedance curve and listening levels my not appeal the masses except I can listen all night long and not want it to end due to listening fatique or any other reason such as that.

For me, a little extra mid bass (or a little underdamping), combined with the falling FR through the presence region mentioned by Duke, defines "smooth".

Vandy's model 2 meets the description pretty well, so I assume that this is what you're after.

I own the Verity Parsifal/Encore and it's a textbook example of this. FR falls app. 5db (anechoically) from app. 2khz to 5khz and the bass will, in most rooms, be a bit elevated between 75hz and 150hz. While this may horrify some folks, the result is IMHO very, very musical.

Many of the older Sonus Fabers also feature this tonal combo, but the Cremonas, while retaining a bit of the mid bass bump, are much flatter through the presence region than were some of the vintage models.

I have owned both speakers for many years, so this post is based on extensive personal experience. However, as always, YMMV.

Good luck.

Marty