Controlling the cross-sectional characteristics of moving tow lines by providing monitors and positioning devices so as to improve stuffer box crimp uniformity. The mass distribution profile of a filamentary tow band is continuously measured as a function of the width of the band, and the information obtained is used to change the position of the band when a deviation from a predetermined value is noted. Positioning devices are used to carry out the movement or adjusting of the band in accordance with a signal so as to correct the deviation and present to a crimping apparatus, such as a stuffer box crimper, a band having a uniform mass distribution profile.