This invention relates to a system and process for modifying the resulting properties of a flash spun plexifilamentary film-fibril web and spunbonded nonwoven sheets made from such webs. In particular, the system includes specifically designed letdown chamber arrangements wherein the solution of polymer and spin agent transitions from a single phase solution to a two phase solution in the letdown chamber prior to spinning at the spin orifice. The method comprises altering the configuration of the letdown chamber to alter the properties of the web and the sheet products made therefrom.