Ok, as stated above, the 'Feel' setting under the SSP button is what you want to use. It basically raises all the frequencies you want. If you completely clipped the rear speakers out, it'll be a bit more difficult to get the mids you want out of the stock speakers, but since you're using a 12 you can compensate a bit for this.
You can use the fader control under the audio options menu to 'increase' the output of sound to either the front or the rear speakers. This will mainly work if you clipped the rear speakers completely. Before I put my second amp in, I had this set as REAR +2. I also had the Treble setting to +1 just get eek out a bit more since I was fading most of the sound to the rear.
From there, it's time to play with the amp. MAKE SURE the crossover filter is set to LOW PASS. If you have seperate adjustments for low pass, i.e 30hz-500hz you'll wanna find a spot right around 200, maybe 250 depending on the size of your sub-box. It will allow enough overhead on your sub to reach a higher frequency, and 'seemingly' giving you a phantom midrange. To AUDIOPHILES this isn't the way to go, but for most this should be sufficient.
Gain your amplifier accordingly so you don't blow your sub, and if your line out converter allows adjustments, set it so that it's at the peak it can run without adding distortion into the line. If you need further assistance, feel free to PM me, or as you can see I have both yahoo and aim messaging services.
