A speaker can be made to sound smooth by designing in a frequency response dip in the lower treble region (somewhere around 2 kHz to 5 kHz). The Silverlines mentioned by Zapper and Bondmanp often have a pronounced dip in this region and I think that Shahinans do as well, and a lesser dip is present in many British speakers (and I would guess something like that is present in most of the other speakers mentioned in this thread that I'm not familiar with). Maggies have a broader, gentler lower treble dip that smooths the presentation.
As Mrtennis points out, there are tradeoffs involved. To my ears, a speaker with reduced energy in the lower treble region is lacking in upper harmonic energy on many acoustic instruments, and so it doesn't have as much "life". I find myself wanting to turn the volume level up louder to hear the harmonic richness that I expect.
I used to own Quad 63's, and would not describe them as an especially "smooth" and forgiving speaker - they could sound somewhat aggressive with some program material. Quad 57's are smoother, and I haven't heard the latest Quads.
Speakers that are free from peaks in the 2-5 kHz region (whether on-axis or off-axis) are likely to sound fairly smooth without trading off too much upper harmonic energy. But if you want a speaker that sounds smooth even with a harsh recording, you'll need a speaker whose response dips signiifcantly in the lower treble region.
Duke
