Thank you my friend, I will talk to Mat at Sound World and have him dial me in. he does not owrk this weekend so your help gets me started. So with a separate amp I will have no need for a 12 ohm sub as it seems they just are not made. The new amp will be like 4 ohm?
There is a way you can wire the sub and new amp to the headunit. You would probably need to take out the NAV and plug in a few wires but it's usually not an expensive job at a car audio shop. I also recommend going with a single twelve inch Alpine Type R or Rockford Fosgate P3 sub with that Alpine amp. The subs are affordable and you can get a ported box for the sub made pretty cheap. The sub will be loud enough to hear just fine in the cabin.
You're welcome! Usually subs have their own amp but there are multichannel amps that can power for speakers and a sub. The problem with the LS is that it's hard for bass to pass through from the trunk into the cabin. The Alpine you mentioned is the eight inch one right? If so it may not need quite as much power as the ten or twelve inch Type R's.
If you use it on your factory amp it would probably kill it. I would highly recommend a separate amp for the new sub. You can keep the factory speakers and the speaker amp while having the aftermarket sub/amp at the same time. It's what I did on my SC300. The ML sound system isn't that good in my opinion so I'm changing everything out on my car, but that's only because I got some speakers really cheap.