For a budget of RM400, i suppose you meant that includes installation and wirings. If that's the case, it would be wiser to save up more before getting an amplifier. Installation charges range from RM50-150, cabling would roughly need RM50-100 for budget ones. And there's little left for the amplifier purchase.

Well... for one, you are using 5.25" hence the lack of midbass from it, most 5.25" will suffer from a lack of midbass due to the fact it's a small size. Maybe upgrading to a 6.5" front component would be a better option.(but again, you'll probably need an amplifier to get the best out of it)

Hm... well... if you don't want to do a whole piece for inside the doors, you can just get one piece, cut it into 2, and place it over the speaker hole location. A piece of 15"x15" would be more than enough to stiffen up the speaker mounting location for less flex and better bass response. But ultimately, a 5.25" is still a 5.25", you'll only start to get more decent midbass response from a 6.5", and if you still feel lacking, you'll probably need a subwoofer to take care of the subbass.

Well... i'll probably be introducing "speaker kits" for RAAMmat soon, whereby it's 2 pieces of 12"x12"(1 square foot size) RAAMmat, perfect for most speaker openings.

It's applied to all accessible sheetmetal around the speaker opening, absorbing and eliminating noise causing vibrations around the area usually caused by the mid/bass. It provides a solid non-resonant speaker baffle that eliminates distortion and will help improve mid-bass performance from the speaker that's mounted onto the treated area.

Comes with self-adhensive, don't have to use heat-gun to apply, just clean off the area, peel off the backing tape on the RAAMmat, and stick on! Installation can be provided at a small fee also

Well... RAAMmat works the same way as Dynamat, but it's slightly(very slightly, in fact) thinner than Dynamat Extreme but it's waaay cheaper in terms of cost/square feet compared to Dynamat Extreme.