with my sub, i have a box that is about 30cm deep x 60cm wide and about 60cm high. its only 25 litres (which is all my sub requires) and doesnt take up too much room, its kinda a triangle shape to match the rear of the seat.. it is best to put your sub in a box if you want good bass, even if they say they will work as a "free space" sub.
your best option if you want bass, but stacks of room in the boot, is to opt for either 6x9 or 7x10 speakers in the parcel tray (be sure to hide them as best you can from prying eyes) or get some 6" splits in the front doors. which ever way you go, if you take either of those two options, your looking at around $400+ for the speakers but the bass you get will be well worth it. kicker or jbl splits are awesome, but i'm sure some of the other members here can recommend other gear.
however, if you want a sub, and you dont mind losing a bit of room, go down to a good car audio store (read as, not autobarn or strathfield etc) & ask them about box requirements for the subs they sell. your best bet would be to run a single 12" sub, or if you can afford it, and the box isnt overly intrusive, a single 15" sub.
i used to run a single jbl 12" competition series sub off a cheap-ish sony 222w (around $280 or so) amplifier bridged to one channel. it worked a treat and i had plenty of bass. then i ran 4 6" 3 ways off a 240w kenwood amp (4 x 60)
provided your kenwood head unit is a decent item, you'd have no problems either running a sub of the amp and 4 6" 3 way speakers off the head unit, or, 6" splits in the front doors, and then something like 6x9's or 6" 3 ways of the head unit in the parcel tray. and then later on, you could upgrade with a 2nd amp if you wanted.
hope this helps you on the path to tight bass