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if you have the premium sound system, then you have subwoofers. The premium system was not standard on all models.
You should see them on the parcel shelf, see the pictures.
The thing is that it seems that the sub setting only works if the DSP setting is on front. If DSP is set to central or all (or similar I do not recall the exact name, but as it is graphic it should not be an issue to find the right setting), then the subwoofer settings are disabled.