I suspect that you've replaced the standard speakers (very sensitive) with less sensitive speakers, and you're running them from a headunit.
If it's a mk2, and you have 6x4s, you won't get staggering bass from them regardless. You can try to increase the bass boost (which I'm sure you have tried), but this will just end up sounding muddy, and lead to clipping (distortion) from too little power.