hmm. well, start with installing the complete set (!), and ensure the midwoofers are solidly mounted, with no air leaks around the speakers. then consider adding sound deadener and sealing over the service holes.
as an idea, here's pics of what i did with my 156. i happened to replace the factory plastic speaker baffles with angled mdf wood 'pods', and went all-out with sound deadener and panels. you don't have to go to such extremes, but it should give you the general idea.
*stripped rear door: note big open service holes (= bad for audio)
http://pic20.picturetrail.com/VOL97/...4/12783104.jpg
*sound deadener on front door: self adhesive Dynamat Xtreme, applied to inner/outer skins
http://pic20.picturetrail.com/VOL97/...4/15165385.jpg
*service holes sealed over: mdf panels cut to shape, sound deadened, then screwed to door, with closed-cell foam rubber edging for gasket; midwoofer mounted on mdf pod
http://pic20.picturetrail.com/VOL97/...4/15363285.jpg
GT . 3.2L V6 . Q2 . Kyalami Black - Red . Eibach . KONI . ATP 19" . TOYO Proxes4 . Mille Miglia exhaust
155 . 2.0L 8V TS . Enkei 17" . Bridgestone RE001