The stock sub is actually a 2.5 ohms DVC, but they're not wired at the sub in series or parallel. The plug to the stock sub actually has 4 wires and each voice coil is getting its own channel from the stock amp. Not sure how the stock amp is internally wired, but I would think it's still 2 channels for the sub. Just for the **** of it, I removed a stock sub from one of the enclosures I used to modify and sell, to see if anything useful could be done with it. I wired the voice could in series for a 5 ohm load and hooked it up to my Alpine mono amp that's powering my aftermarket 10" sub. I knew the stock sub wasn't going to be able to handle much more power than the stock amp was sending it and I confirmed that VERY quickly. The thing started popping with the bass even turned down drastically!
Don't waste your time trying to power the stock sub with an aftermarket amp. You will most likely blow the **** out if it quickly. The stock sub is mated to the stock amp and that's really all the power it can handle. If you're going through the trouble of adding an aftermarket sub amp, then at the very least, replace the stock 8" sub with one that can handle the power of the aftermarket amp. However, I've stated several times on this forum that IMO, an 8" sub just can't move enough air in an SUV to have any meaningful impact. I would recommend stepping up to a 10" sub, and I'm personally running a pair of 10's these days. One is in my modified stock enclosure, and the 2nd 10" is in a small sealed box that can easily be removed if I need my whole back hatch.