If you want to change the impedance of a driver you change the gauge of wire used for the voice coil. Thicker wire = lower impedance. Using thicker wire also decreases the efficiency of the driver, however, lowing the impedance allows more current to flow so you can get more power into the driver. If you want your speaker to have more impact than your neighbors the easy way is to lower the impedance. It's always good to remember that impedance is frequency dependant and also dependant on the quality of the driver and crossover design. A poorly designed "8 ohm" system may actually have dips below 4 ohms where a "4 ohm" system may never dip below 4. Most systems are 8 ohms because it is a good "middle ground" for providing a good stable load for most amplifiers and a good compromise for keeping the most copper in the gap of the motor. As a general rule, the more copper that stays in the gap the better.