The best place for ULX settings isn't the Source server server.cfg, it's in ULX's <your server gmod path>/data/ulx/config.txt
If you don't have one, copy the default one from <your server gmod path>/addons/ulx/data/ulx/config.txt to the path above, then you can edit that.
I also believe there's probably a way to edit the setting in XGUI too.