Sorry for all these audio posts everyone.
As I have decided to ditch the stock HU, id like to know some info about the factory amp before I get the new unit. What are the specs of the amp (channels, watts, etc) and what speakers do they power (just the rear or all). Would it be right with an aftermarket HU? I also read somewhere that if you upgrade the positive and ground wires on the amp to 8 gauge wiring, you get more power out of it. Is this true and recommended?
Would it be advisable to upgrade the speaker wiring as well?

it alpine, 150 watts, and powers the parcel shelf sub and speakers. thats about all i know.

Thanks mate. Anyone else wanna confirm?

Well from what I know it's an Alpine 4 channel 150 watt amp. Good enough for a factory set up but not much else.

I would suggest you just upgrade the amp to something with a bit more power in it. You are also correct about the wire gauge, the thicker the wire the more current that can flow through. This will give you cleaner sound provided the system can utilize the cable, no point in using 2 gauge wire for something like a factory system.

Also if you are re doing the wires and such upgrade the inline fuse to something a bit larger maybe like a 100A or 120A this will allow for a little bit better audio.
I was running a 60A inline fuse and I went up to a 100A made a notable difference for something so small.

at least in my AU, the factory amp and sub sound good for what they are

having just ripped them out, and replaced with twin 8" boxed subs with 300rms monoblock amp the difference is far from what i thought it would be.............

Not happy on how it compares to the original ? May take some time to fine tune it? It took me ages to finally have a budget system that sounded decent.

Thanks man.
Iv been looking around and people say that the factory amp is a 200 watt 4 channel. Alpine 3554 or something.
It seems to be coping with the new speakers alright but i could be wrong. What would be the recomended amp? Also, if you raise the size of the fuse, doesn't that reduce the protection of the amp in the event of a short circut?
Anyway my next problem is the RCA setup on the new head unit. It has 4 rca channels, 2 for the front and 2 for the rear/sub, which leads me to my problem. I have to choose between rear speakers or the sub. Is there a solution to this so i can have both run to the amp?