I did the ICE changes in stages, based on my budget and affordability.
What I did was:
Stage 1: Changed all speakers, front and rear to 2-way speakers. same size (not the component set). I think the brand is infinity, i think so as it is like 9 years ago. No amp whatsoever. Results: just clarity improvement, no bass whatsoever.
Stage 2: I diy soundproof, just stage-1 sound proof 1 layer on outer door panel for 5 doors and boot. Sounds better, without any hollow and ratlling. No bass still but clarity is good when you adjust the HU treble and bass setting. More towards the treble so you wont get the too-high freq sound.
Stage 3: Added underseat sub. Gotten 8 inch back then (Mohawk)... but I still prefer 10-inch ones.. anyway, this sub helps to give the boost of bass. So fine-tune the HU bass setting and also the underseat woofer setting to your liking. This may be a few rounds setup tasks until you get the sound you like.
All the above is still on standard HU. after about 8 years, the underseat woofer blown early this year.. so gotten a new MBQ 10-inch. It sounds different, but since recently, the android HU is quite good, so I gotten one early this year.
The android HU has more sound setting, where you can further tune the sound instead of just treble and bass.
So far, it is good enough for me as I am not the superduper guy who turn my car into a bomb-bomb unit.
Hope good info for you.
*some / most accessory shop will just install for you and there are sound out.. no setup be done, unless pay them kua.
*my friend did ICE on his car, (not Alza) spending >1500 on HU, door speakers, amp, ended not to his liking and he said my simple setup way better than his...he regretted.
Thaxs mate. Really appreciate your efford to expalin in detail