You need the ohms at the crossover point. The speaker needs to have an impedance sweep done and then measure the ohms at the desired HZ you want to crossover at. That's where a DATS system comes in handy. If you want a really good crossover you could download the free Jeff Bagby PCD program/ excel spreadsheet. It's did me right. But you'll need .ZMA files that DATS will spit out if you ask it to.