Underseat Active subwoofers?

[Brummmie]

I have a Merc Vito, and the stock sound system is awful.
I have got some Alpine Component upgrade door speakers, but the head unit is struggling especially on aux input from iphone to provide any grunt.
Next step i think is to amp the front speakers, and a bass unit, its got to go in the cab as a big sub in the back will get a Chainsaw/branch/GSXR through it.
I want to keep the standard head unit, as its discreet and steering wheel functions plus phone kit are all integrated.

Can someone recommenced a good (as it can be) underseat active sub?
What about these?

http://www.ebay.co.uk/itm/Kenwood-KSC-SW11-Kenwood...

[Butter Face]

I've just bought the exact same one to fit in my car, not got around to it though.

Supposed to be a pretty decent amount of sound for the size!

[gmaz]

I had one of these, which worked well enough.

http://www.ebay.co.uk/itm/Pioneer-TS-WX120A-Under-...