Speaker is the device that produce sound. So what u hear is sound, means from speaker. So speaker determine the most. Under same player, same place (in this case, front door), change speaker alone can make a big different, in high and mid range (vocal), but not necessary in bass, why?

For deep/soft bass (those thumbing bass), 1st condition you must have a large room even ur speaker is small. 2nd condition the room wall must be rigid. So the front door actually can't fulfill these 2 condition. Because the door metal is flex, when bass hit, the metal will vibrate/flex @ the same frequency as bass, so eventually cancel out each other. Furthermore our saga door thickness is thin only, not enough room for those low frequency reflection. So what u can do is only improve the condition, in this case by dampening material, which make the door panel become rigid and less movement when bass hit, so the bass will be back. Even stock speaker also will give u stronger bass. If u need those soft/deep low, the only thing u can do is by subwoofer only.

Our Saga main advantage over 'bootless' car is, our trunk act as a big speaker box the reflect the bass to the front. Without subwoofer, if u put the same speaker @ rear and front with same bass setting, actually u can hear how deep the bass can produced by rear speaker.

Speaker unit alone without any enclosure/cabinet will not hear any bass even the very big diaphragm is move like mad men, you can do this simple test your own.

The main point of above crap:
1. Need soft/low note bass, subwoofer is the only way to go.
2. Want stronger bass with stock player and speaker, u need dampening.
3. Change speaker alone might change the sound signature alot (vocal and highs, as this region fill the most in audible spectrum), but not necessary bass.
4. Change HU alone without changing stock speaker is = waste money.
5. You can change speaker to try 1st.
6. ICE is expensive, spend wisely.

No sifu teach me, just my self explore in this item these years (through my own setup and google). Some of it could be wrong, and i know what is the best sound i want, honestly not my current crap setup, becos no money.