@ RSM : If you are using the OE HU, then it wont have the required pre-outs.
What i came to know from other Punto users, the OE HU has sufficient power to handle more powerful speakers.
What you can do is install a line-out converter with a 4/7 band equalizer. Install a good 4 channel amp, go for nice component speakers in front & in rear too, since you are saving on subwoofer!
And which is the good brands for speakers?