There is no GUI setting to control the output format. This is because the tools need to be able to read the results...
You can probably specify "-f psfascii" in the userCmdLineOption field in the Setup->Environment form (I've not checked whether this works - it may not). You may also be able to specify the .cdsenv setting:
Code:spectre.envOpts simOutputFormat string "psfascii"
but again, I've not checked that.
Andrew.