Just gone to Slackware-15.0 with xfce-4.16.0.
There are two screensaver programs in X as installed

• xfce4-screensaver, which is started by default and comes with 3 or 4 very sucky choices of screensavers.
• Xscreensaver which is not started by default but comes with a better choice.

I have currently resorted to configuring the autostarted xfce4-screensaver to go and play blindfold in the traffic, auto-starting Xscreensaver (which pops up it's settings dialogue every time) and configuring that to show a tranquil screensaver. I can't seem to find where either store the screensavers.

Can I improve on this by installing useful screensavers for xfce4-screensaver or adding a switch to xscreensaver not to set up every time?

I have one question that may appear to be from left field: When you set xscreensaver to autostart, did you point to xscreensaver to xscreensaver-demo?

I ask because xscreensaver-demo is the configuration dialog. It you start it, it will start xscreensaver, but can start xscreensaver directly, withoutout calling xscreensaver-demo. I have done it on many systems that don't come with xscreensaver as the default.

Like I say, just a shot in the dark.

^ Yes, xscreensaver can definitely be made to start without popping up a settings dialog.

About the choice of screensavers:
My distro (not Slack) has separate packages for additional screensavers, they're recognized by all *screensaver programs (All ${DE}-screensaver programs are just forks of the original xscreensaver).