dbdrive pl12d4 and americanpro v.s 1409

Alright, I will start off by saying that I know pretty much nothing when it comes to Ohm loads and technical details of that nature so please bare with me. Ok, I have each VC on the sub hooked into its own channel on the amp. As a precaution I took the left RCA cable and split it with a Y-cable, from radioshack, so that I don't have a stereo signal running to my single DVC sub. This set up is working great but I originally had only one VC bridged at the amp. The original setup was much louder as far as the amount of sound that the speaker produced but the amp would over heat after about 45min to an hour full blast. Now my problem is that with both VC's hooked up the amount of sound coming from the sub has diminished(I can't hear it from as far away as I could before) but is shakes my car twice as much and I can feel it in my chest better. I am sure there is some kinda reason for this and that is perfectly fine but I have a feeling I could wire this thing differently and achieve the same sound quality I had before without the amp getting hot. Here are the specs for both the sub and amp: Features of the DB Drive PL12D4 include:     * 3" dia high temperature 4-layer voice coil     * Extreme excursion motor structure & suspension     * Multi-layer treated paper cone     * Layered UV treated foam surround     * Dual spider linear design     * Strontium (SR) magnet with attractive rubber boot design     * Attractive matte black anti-resonance die-cast aluminum basket     * Integrated tinsel leads for high excursion     * Machined push-button terminals     * Rigid dustcap design 12" 1500 watts Dual 4 ohms voice coils     * Sensitivity: 89 dB 180 oz magnet Features of the AmericanPro VS-1409 include:     * Four Channel Bridgeable Amplifier     * Power Ooutput: 320W x 2CH. = 640W @ 2 Ohm Max.     * 220W x 2CH. = 440W @ 2 OHM     * Bridged(Mono): 640W x 1CH. = 640W @ 4 OHM     * MOSFET DC-DC Switching Power Supply     * Built-In Dual Protection: Short Circuit/High Temperature     * High/Low Adjustable Level Input     * LED Power Status Indicator Here are some pictures of the Sub/Amp and Box. http://notw.110mb.com/audio.htm

I checked the Probox website just to look at specs for the box and the instructions for maximum sound in that particular box were to set the "Bass Boost" at +12dB to +18dB at 45Hz with the LPF set to 90Hz or lower. I set my crossover all the way down to 30Hz because I really don't hear much of a difference when I turn it up unless it is turned way up. Anyway that amount of sound increased a bunch but it still doesn't hit like it did with one VC wired instead of two... I really would appreciate some help here because I am not going to mount everything and make it look right until I have the thing wired correctly.