Try going to Santolan (A. Rod) in Pasig. Look for Mach 2 Audio or Mickey's Autosound. They know what they are doing there when it comes to car audio. If you are not too particular about sound, maybe pwede na yung subs sa quiapo. )
Also, there's a thread here that recommends not putting subs anymore for the Strada. Just add sound dampners to the doors, speaker baffles, better coaxials for the rear and separates for the front plus a 4 channel amplifier to power all speakers. Mas lalakas palo ng bass.