I wouldn't reuse the Shaker 500 amplifiers on aftermarket speakers and I highly doubt they have the ability to be bridged. Another downside to the Shaker 500 factory amplifiers is that they are phased weird. Naturally, the phase issue only comes into play if you are using an aftermarket trunk subwoofer in addition to the stock Shaker amplifiers and subwoofers.
IIRC, the shaker 500 subwoofers are dual voice coil and your Pioneer slim mounts are not. I wish you lots of luck if you try to reuse the factory amplifiers on your aftermarket speakers. You may find that you made things sound worse instead of sounding better.
