It sounds like your subs are set right. I'm assuming your subs are 4 ohms each ( this may not be true) but if you wire both positives to the same positive on the amp and same for the negatives you have them run in parellel to the mono position on the amp. The amp will see a 2 ohm load and perform at its maximum. This will send 800 watts to both speakers and they will each see 400.

This will be plenty loud. I know subs that get 300 watts and you can hear 1/2 a block away. Check the gains on the amp, you can increase its sensitivity.

Also, the converter may cause a problem, Im not sure ive never used one before. That signal will be everything the rear speaker gets and if your bass is turned down on the stock stereo then the bass will be low to the amp and subs. Im not sure of this, try the sensitvity thing first.