I installed the little JVC 6" powered subwoofer last night. I put it right under the driver seat. It does the trick in just filling in some extra low-end bass. While it won't thump your ear drums, since it is sitting right under me, I can feel it nicely. It also has a cool little remote, with a light button to show when it's on, and a volume control. Again, they're not going to hear you down the block, but to me it adds what is lacking & doesn't take up any space.
I think this is about the only powered subwoofer that would fit under the 4Runner's seat, as the seats almost right on top of it.
As far as the installation, it took about 2 hours. Much thanks to whoever posted all the great pictures of taking the rear inside panel off to get to the factory amp- this really helped!
As per the Crutchfield dude (those guys are great!), I tapped right into the 4 amp outputs to the factory subwoofer. (It's a dual cone.)
Heard something interesting from Crutchfield. They have all the schematics, etc. for the JBL Synthesis (I have them too, thanks to someone posted them on here). He said that ALL of the lines running to the amp from the head unit are data transmission lines of some sort, and therefore you can't get signal off of them. In fact, if you tap in, it will likely mess things up. A friend who had a local chain of car audio stores, told me the same thing after he talked to an acquaintance who was a JBL engineer.
He also mentioned that after-market head units can mate up to this amp now, but you have to buy the $150 wiring harness. It used to be supposedly "impossible" to do, proprietary, etc., but some engineers figured it out. Just ask Crutchfield!
Not that I want to change the head unit or anything. I'm satisfied for now. I've got a volume control on that blaring center speaker, and xtra bass vibrating under my butt! (although maybe I should put one under the passenger seat too, to "warm" my wife up)
I was surprised how small & light the factory amp is - that thing really runs 10 speakers & produces the surround sound!? (Crutchfield said the amp produces surround sound, not the head unit.)
In any case, I'm pretty happy with this little, discrete unit, for $150!