I could never really figure out the DefIcons prefs tool. The other way to make this change is to go into SYS:Prefs/Env-Archive/Sys and then get information (Right Amiga+I) on the file def_lha, then change the default tool to Unarc.
If any of your LhA files have their own icons (i.e., non-default icons) they'll need to be adjusted independently.
