Andy
The easiest way to do this (what I may end up doing) is to keep the standard rear 6x9's, dis-connect the cable going to the sub part of 6x9's (havent checked the cabling yet but they are 2 seperate speakers, sub & mid/high), use a line level convertor to connect to an amp or get an amp with high level inputs & connect this to your sub.
I am not sure of the quality that this will produce as I have never used line level convertors, usualy, the more electrical stuff you have in the chain the more interference you will get, but you definately have the means to get more bass!!!
Also, replacing the rear 6x9's in my opinion is pointless, you don't sit in the back of your car do you? & you really wont notice any difference as the new sub will be doing a better job the the little 6x9's would have.
If you do go down this route, please let me know as I would be interested to know how well it works.
Cheers.
Stuart