Indeed they do else you wont get any bass at all!!!!!
Make sure the enclosure is the right size, iirc theres an faq on here some where, if not then
www.carstereo.com has loads of faqs and information on everything ICE related. Helps you understand the principles behind speaker design.
Plus don't forget that subs can be mono, whereas the 6X9s need proper stereo placement, or all the sound coming from the back will be totally mono. Left should be left and right should be right. Thats another reason why 6x9s shouldn't go in parcel shelves! (yes I know ours are there but we don't have the money for componants yet! plus we might be gettin a new car, see the other thread!

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Jon+Em