Assuming that both subwoofers are working fine: check what voltage output your headunit has (RCA output voltage) and match it to your amplifiers input sensitivity (GAIN) eg: if your headunit has 4 volt outputs then on this particular amplifier your gain knob would be completely to the left as if in the off or 0 posirion.
if u are not sure of your headunits output voltage you could check the manuals specification page or send me the barand and model number of your headunit and I will check it for u.
hope this helps. let me know if I can assist you any further.
Answers & Comments
Assuming that both subwoofers are working fine: check what voltage output your headunit has (RCA output voltage) and match it to your amplifiers input sensitivity (GAIN) eg: if your headunit has 4 volt outputs then on this particular amplifier your gain knob would be completely to the left as if in the off or 0 posirion.
if u are not sure of your headunits output voltage you could check the manuals specification page or send me the barand and model number of your headunit and I will check it for u.
hope this helps. let me know if I can assist you any further.
.......andy.......