First off, make sure you are using the dynamic impedance of the driver at the cut-off frequency you desire.
If a driver is listed in a catalog as, say, an 8 ohm driver, if you hook up your multimeter across the voice coil,
that is roughly the reading you will get. With the driver playing music, the actual impedance will vary. It can
go as low as the static resistance, but will more likely be several hundred ohms at higher frequencies.
You need to get the impedance plot...