Should you have said in the first place what you have (3") my queery would have been much different. Yep, you need help to your sub. Try to make the speakers as complimentary as you can. Paper to paper, etc. and even the same mfg if possible (that never seems to happen). A seamless crossover point is a primary concern. There is tremendous power in the 100hz - 500hz range. You may need a greater cone area than you indicate, but 6-8" is about a 16" dia speaker which sounds about right. Keep us posted as I'm facing the same kinds of problems with my NSB's. I hesitate to recommnd better 8" for the job as it usually hurts the pocket book. Zene