Pag bumili ka ng isa pang sub, di mo na magagamit ang dati mong box kase mawawala na sa design, mas maganda kung i mamaximize mo yang nandyan na at focus on a good head unit muna, then pag wala ka pa nung seps para sa harap, sunod mo na rin. For spl system, you have to get a good amp(mono amp) for these, pwede nga yung 2 sub na gt5 s12 in a good tuned box like 38hz for spl, then surely, at least 800watts * 2 ohms monoblock if your sub is jbls12 single 4 ohms, unlike V-12 amps kase, mono or class D amp are better driving subs, tipid sa kuryente at mas solid yung bass, pure kase na bass lang ang lalabas sa subs mo.
Head unit, beter hanap ka ng at least 4volts 3 rca pre outs like yung bago ngayun na pioneer 6350, or, pwede din yung iba na 2 volts lang na may rca out, then gamit ka na lang ng active crossover, sure budget dyan around 30k minimum kasama na yung woodworks sa trunk, necessary wirings, electrical upgrade, cables, amplifiers etc, because, you want spl, he he, big three upgrade and a 2 farad capacitor for sure, maybe ad it to your budget, if you stay on your current set up, pwede rin yan, palitan mo na lang yang v-12 mo ng mono amp at least 300watts rms * 4 ohms para sa sub at gamitin mo na lang yang amp sa front and rear speakers, head unit syempre kung wala ka pa..
Recommended shop, ehem, kung around taguig ka lang dito na lang sa shop namin sa taguig, if not, marami nababanggit na car audio shop dito sa forum e, better ask first na lang muna for prices baka madugo, err,