I have the stock Becker CDR-220 HU, with dash and door speakers. I added rear speakers which were originally powered from the HU, I then added a 6 channel amp from a 996 to power the rears at 40W vs 15W from the HU.
In my opinion, the rear speakers make a huge difference to "fill out" the sound, especially after adjusting the forward/back fade/balance. I would install rear speakers first, because I think they make the biggest difference. I was not able to notice any difference going to the 6 channel 996 amp, however the 6 channel amp gives more flexibility if/when I upgrade my speakers. However, I have been happy with my stock HiFi sound plus the rears. I use my HU almost exclusively on AUX with an iPod hookup, so I have never felt the need to upgrade the HU. I'm sure a newer one could make a difference, but I'm not enough of an audiophile to give up the OEM look of the Becker HU.
At the end of the day, the speaker/HU/amp part is very subjective, but I would definitely recommend adding rear speakers.
Hope that helps!
