I ended up buying 8 inch Pioneer Shallow (free air)subwoofer from ebay(see below). I had a 10 inch JL Audio, but it cannot fit underneath next to the stock speakers. I need the space for the baby stuff and stroller that's why. The BMW 135i due in May is our non-baby car.
Amazon.com: Pioneer Premier TS SW841D - Car subwoofer driver - 120 Watt - 8": Electronics