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I doubt the stock amp will have the clean power and amount of power a Hertz sub will want.. I think you'll have to end up adding an amp anyways so I wouldn't worry too much about impedance but if I had to guess its probably 4ohm.
Thanks for the info. After thinking about it, I'm going to redo the whole thing. I'm pulling the MA speakers I just put in and I'm going with German Mastros in the doors, a Hertz Sub and Hertz amp.
I like the way you think when you redo things haha. You'll definitely be impressed with that setup over the MA speakers and even more over stock. Keep us posted on the install!