Not quite it depends on the cars resonant frequency also, if your box is tuned to 43hz its not likely to get loudest at that frequency. What the box is tuned to determans what frequency the sub moves the least, in your example it will move the least at 43hz.
All turning the Db up will do is make it so you dont have to turn the stereo up as much to get to full volume, it depends on the car and box tuning as to which frequencys will be loudest.
if your using an amp like rockford, it should say on the birthsheet, or if there is no birthsheet and your using a D class amp it should be around 70-85% or may say in the manual.