For 6 inch speakers, it's hard to go past brands like Boston, Alpine, JL Audio, MB Quart, or if they break the budget, Kenwood and Pioneer make some pretty decent stuff. I am not sure what the factory amp that runs the sub is, do you know any details on it? You will get better sound with a box, but they do take up space so you will have to decide if you can afford to lose it. As for free air 10 inch subs, Kicker make a pretty decent one which I have seen going cheap at JB HI-FI down my way. If it was me I would get some details on the power output of the factory amp, they are generally piss weak, but hopefully for your sake I am wrong.
Black EB S-XR6 5 Speed Lukey Exhaust Lowered AU2 Tickford Wheels
Alpine CDA-7892E,Alpine MRD-M500, Alpine MRV-F303, Alpine Type 'S' 12" Subs x 2, Boston Acoustics RC620 and CX-6