I don't know of any dual voice coil 6.5 inch subs. Keep thinking someone on one of the forums will find one & post it but not yet. Without one, the only real choice that uses the existing location is to add a single voice coil sub plus a sub amp.
I got the RTFM license plate a long time ago - back when only computer dweebs knew what it meant. (Not that I'm a computer dweeb or anything like that, of course.) For anyone who's never run into the term before, it's an emphatic way of saying Read The Manual.
I have searched for a dual and there is not one to be found. JL, Kicker and Rockford make 6.5" Subs. The amp on the E is a 6 channel amp. 4 channels going to the doors, and 2 going to the sub. 1 for each voice coil. That means each voice coil is getting 45 watts to it. I have not found out yet if the factory amp is bridge able, as alot are. If you don't want to have another amp, you could run an aftermarket sub to one channel of the amp, and leave nothing on the other. The JL would probably best for this situation. Or run 2 after market subs. 1 on each. I am waiting for the Rockford 6.5" sub and going to run a Rockford 250.2 amp to it.
alright, first of all, when it comes to the stock sound system in the element, if your looking for bass, replacing the stock subs is a ridiculous idea, why not upgrade to aftermarket subs, and of much bigger size. I am running a 2000 watt Lanzar 2000 XLB with a 1.3 farad powerbass capacitor to two 12" competition grade explodes. And i will say it pounds like a mother. Im getting new speakers this summer to replace the pioneers i put in last summer. Ultimate goal is to make the element a rolling studio of amazingly clear music