Well if you want to get louder on the meter, it is usually much easier if you tune higher, thats why you see some of these guys with 170's burping at 55-65Hz, though that kind of box would be poop for a daily application. For daily tuning around 30-35Hz the only thing you can really do is use better subs and more power and/or build different type's of boxes such as a Single reflex Bandpass, or a 6th order bandpass, can make you louder with the same equipment.