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...