In newer versions of Ubuntu (and Linuxes) xscreensaver has been replaced with gnome-screensaver. The same screensavers are supported (with some exceptions, I guess), but there is one thing that is lacking: per-screensaver configuration. The author of gnome-screensaver thinks that this is not needed.
There are some ways to get around this, among them: replace gnome-screensaver with xscreensaver (this has some side-effects that may not be desired) or install xscreensaver alongside gnome-screensaver.
If you are comfortable with editing configuration files and reading man pages I would recommend the following way: find and edit the desktop file of each screensaver you want to configure, using the screensaver’s command line options.
Find out where the desktop files are located, read the man page and then edit the desktop file.