No such thing as underpowering a subwoofer, if your not going to be in competitions then that extra 600 watts per speaker wont be heard but probably seen on a meter. You answered your own question, if you want to run it at 4 ohms and make your electrical easier then why not just run the Zapco. If your really worried about sq you would worry about your subs last and not even worried about how much power you give your subs. The box will be the best overall factor in how much power you can give a subwoofer, if you want to give them 1100 watts then build a box around that power level.