Well, perhaps this train of thought is going to be unpoular, but I think you're mad using a Nak head unit and then playing it through £75 vibe speakers. If you're going to use what are, albeit not bad but entry level speakers, why use a Nakamichi head unit? You may as well use an old JVC....
My suggestion would be ignore the rear speakers - disconnect them if necessary - and concentrate on getting a decent set of components up front, swiftly followed by some decent amplification.
Then get yourself a sub and amp to add some decent depth to the music and filter off the sub-bass from the fronts, which in turn will allow you to drive them even harder.
Absolutely the last thing I would do is spend money on rear speakers....