Rca voltage multiplication or not ?

I have a new premier deh-p750mp that has 3 pairs of rca outputs , front , rear and sub. this deck says 6.5 volts max output . the front and rear seems to be that but the sub is much lower like 2 volts . I also have a sound barrier commander 5sw which has an input for sub and out put for sub.it also has a control for sub volume and frequency . the voltage output on this is 3 volts(manual says so). now when i run the output from the deck sub through the commander then to the amp , it sound much much louder . can someone tell me what is happening here ? Is it that the deck voltage gets multiplied by the commander voltage ? what is the resulting voltage output ?"I'm So Loud It Hertz"

No, the voltage is not being multiplied.  The amp is getting the output voltage sent by your "commander."  What is probably happening is there is a signal boost occuring because of level mis-matches between the HU output and the "commander" input, or the "commander" output gain is set higher than your deck's output.  You say the unit has a "sub volume" control?  That would be the gain control for the output.  If the unit is boosting the signal too much, thurn it down.  :)