For the 'average' owner the factory fit sub isn't that bad.
I've retained mine even though I have changed the door speakers.
Yes it does lack real 'oomph' for those who like thumping base,
but then most of those type/sort of people own Escorts/Fiesta's/
Pug's/Clio's and the like.....and 10 trillion watt amplifiers...LOL.
However, it does have it's limitations, and a bit more of that
baseline would not go amiss to be honest.
Think of it more as a 'backing' subwoofer.....it does it's job well,
without being over obtrusive.
Shortly i'll be trying out one of
Evil's suggestions for a cheap and
simple fit self contained powered sub. A 100watt powred sub that fits
into the middle of the spare wheel....from out of a Mazda6 !
even if it sticks up/out it's small enough to fit in the boot on it's own.
(Checkout
www.evilution.co.uk for more info)