Wow! Thank you for all the response! Here's what im planning to do based on my discussion with a fellow SCPH.

1. Replace HU from stock clarion (with cassette pa) to any authentic Pioneer HU (cost around 4500 new)
2. Add NBN sub with built-in amp (cost around 3500 new 2k used)
3. Labot should be around 1k max for HU and sub so im looking at around 9K.

Any comments? I guess this would be my first move muna. Pag di ako satisfied o makulangan, i can opt to change my front speakers first based sa mga nabasa ko dito kaya naman ipower ng 2 front speakers buong cabin.

baka may extra kayong sub or HU diyan hehe. Oh btw di na din daw kailangan palitan alternator ko. kaya naman daw ng stock yun. worse case, may big 3 ako para stable