The first thing to do with regard to the stereo-sound of a GTV (mine being a '99 version) is to plug the holes around the speakers in front and rear.
If you dont do this, then (bass-) air will simply flow from the front of the spreaker to the rear, and vice-versa, and so... no sound pressure.
For isolation, I used the foam that you buy in DIY shops to isolate pipes for heat-loss; flexible, airtight, and easy to chop-up into pieces that fit the holes around your speakers.
This should give you a vast improvement in (bass-) performance, even with original speakers installed.
I replaced the rear-speakers with better ones, and filtered these to only do bass (Pioneer radios have this nice feature... ); happy with the results of both measures taken..
Sometimes, benefits are in cheap solutions