Heaps of places mate, Jaycar has cheap amps and subs, BJ HiFi always has a good selection, Gary's Car radio, Strathfield, etc, etc, there's heaps of places, go and talk to the guys there and do some deals.
if you're after some second hand gear this site has a lot of car nuts selling bits and pieces and even cars.... firesport.com.au
i haven't bought anything off of there though so i'm not sure of the reliability aspect.. i guess a pick up and pay is ok as u'd be able to inspect items.
MY08 2.5i - Satin Pearl White - Sat Nav [TEAM WHITE]
The electronics in the jaycar response amps are hard to go past in value for money...
Some of them have even won car audio aust awards.....
Worth while giving them a listen.
Plus there's a jaycar everywhere!
If what you say is true, then what does the OEM subaru subwoofer/amp plug into. Not the mac sub, but the sub you can buy from subaru. Because I have one and it plugs into an 8pin plug at the back of the headunit.