1) huge amounts of air leaking from under subs.. take em out, get some double sided tape, cover the sides of the sub holes, then make some foam rings, place em on the double sided tape and screw ur subs back on tight. use all 8 screws.
2) since ur rear speakers are at the back and inbetween those and urself u have ur subs, u wont hear ur vocals very clearly. thats the reason y people opt for front speakers/comps.
3) try experimenting urself. eq, loudness, sla, frequencies and that bba, put em all at flat. switch ur sub off. put on some songs and change ur speaker freq's to what suits u best. (or arsalan bhai can tell u what setings to do but trust me, unless and until he dsnt check the setup himself, he cant give u the optimum settings) so take ur car to islamabad.
4) voltage drop was fine. not going under 12V so that means u have good electricals. still big3 wont hurt
Rest best of luck