which headunit do you have?
I have a 1.6Tdi 'S' and have the RCD310 (the big unit with ipod/usb connection in the centre console) comes with 8 speakers as standard with that option and it sounds pretty decent but not enough bass.
I am currently in the process of building an install (2x 10" subs in the boot) to give me the bass, as even in my mk3 golf the standard midrange speakers/tweeters arent bad.
regardless of what speakers/subs you want to put in there are no RCA outputs on the stock RCD210/RCD310 headunits. What I have got on order (from hongkong) is a little 6" extention for the cable that plugs into the back of the headunit, then also a 4 way audio line out convertor which takes 4 speaker outputs and gives you 4 RCA outputs(also a remote for your amps), I'll splice this into the extention as its something I can remove at any time without damaging the main loom.
Once you have this sort of thing plugged into your headunit, then you can connect in as much as you want really as you then have RCA speaker outputs for amps.
I'm still learning
but hopefully by the end of next week I might have something working!