Just to clear up a point here, the slider on the smooth brush button does not control the intensity, but is rather a cavity effect:
“its slider controls the ‘direction’ of the smoothing effect. If set to a positive number the brush will smooth the recesses of the form more strongly. If set to a negative number it will smooth the high points of the form more strongly. If set to 0, performs a full smooth. For example, set this to 100 to fill a valley containing mountains with silt; the tips of the peaks will be retained.”
copied from http://www.zbrush.info/docs/index.php/Brush_Palette#Smooth_Brush
To adjust the intensity of the smooth brush, you need to use Z intensity as you do for Std brush. However, the smooth has Z intensity at 100 by default so you cannot increase its effect without going down in subd level or increasing the brush size.