There should be a dial near that switch that adjusts it, it will have frequencies on either side much like the LPF, but the numbers will be lower values.
If you box is tuned in the low 30's then the SSF should be set near 30Hz, if it is too high then your amp will not send out frequencies lower then where it is set (say 40Hz...) and your subwoofer will only play when you play tones aboze that setting (like the 50Hz you played....) If you have never adjusted it, then I can guarentee it is in the incorrect position and could be the cause of your lack of output.
