I ripped out the two rear speakers. Run an adapter from the connections of those two speaks to a 1500W Amp, output audio from that amp runs to a 750W amp. The 1500 amp runs a 15" sub which is currently sat loose in a bass box in the boot (falls over if I slam on the brakes...) and the 750W runs a set of Alpine 6x9's sat on the parcel shelf.
If you do anything similar you need to fade the speakers to Forward 3, Bass +5, Treble +1. (there is no loudness setting on mine). With the forward fade the volume at 21-24/31 pretty much maxes out the front speakers but they can be heard in unison with the gear in the back.
I plan to change the speakers in the front doors so they can handle the max stereo output, but I won't amp them.
Remote cable by the way, which has been the biggest pain of my life and I've changed it several times now (having had to buy a new battery last week for £80 coz the amps were running when the engine/ignition was off and killed the thing).
I now run the remote cable from the fuse box down to the bottom right of the steering column (that little storage pot can be pulled out to expose the fuse box). The remote cable is plugged in to one of the ignition fuses, I forget which, and then runs to a switch fitted into the top of that storage pot thing you remove, just in case. When switched on a red led I fitted to the ash try lights up to make sure when I switch the engine off I can see if the amps are still running.
Holy wall of text, sorry. But the above hopefully shows how unworkable/difficult the fitted stereo on the N16 is!