Bjorn, i have changed the speakers and installed an amp & sub on my 147. It is worthed the try.
You must know that:
1) 147 head unit does not have a line out connector (RCA), this means that if you intend to keep it you will have to drive your amplifier by the head unit's speaker output (make sure your amplifier has high level input connectors). Otherwise you must buy a head unit as well (rca connectors=better sound)
2) The door woofers are 16cm but they are too close to the window when
its down, (the original woofers have a plastic extension to be far from the window). I installed my 13cm Vifa woofers by making a wooden ring 17,5 cm diameter - 12 cm hole - 2cm thick and used it as an extension.
3) The factory tweeters can be removed by turning them anti-clockwise (hold them from the back
4) There is enough space to glue your crossovers under the arm-rest area on the door cover.
5) The door cover is a pain to remove without harming the door handle. The oval screw cover on the handle must be removed carefully. You can drill a hole on it and pull it out, without leaving any screwdriver marks on the handle. You will have to pay for new screw covers. I glued a big screw on the cover and pulled it out but i had to do it many times to manage it. Thermo-glue leaves no marks, it worked for me.
6) The subwoofer does not have to be large to produce decent bass, just make sure its installed in a fine tuned box. I bought a 8inch Lanzar subwoofer and tuned a 17 lt box for it. Noone believes its that small unless they see it wink I laugh when i see huge boxes taking all the space and trying to boom..
7) If you want to listen in high volumes you must get a 5 channel amplifier, lead the 4 speakers with the high-pass filters on 130Hz (no bass on speakers - no distortion) and lead the subwoofer with the low-pass filter on 130Hz (send the bass to the subwoofer only). A fine tuning is needed here to have perfect results. You will have the volume up to the end and hear no distortion!
I hope these didnt sound Chinese (or Greek!) to you. If so ask help from a proffesional.