you need to ask yourself what frequency you want the subsonic filter to bet set at. if you choose it to be 25 hz like mine then u need to determine how many volts you should see on a DMM when playing a 25 hz sine wave through your CD player (compared to the number you get when playing a frequency at an octave higher, in this case 50hz). if you get 20 volts when playing 50 hz sine wave then u need to figure out how many volts are 24 decibels less than 20 volts. i dont know the math formula u can google it. when u figure that number out you would play that 25 hz sine wave through your system and dial your SSF setting up until you get that number.