Passive Low Pass Filter
If you need to place it before the speaker, go for LC, any R (be it RL or RC) will give you voltage drop and useless power dissipation, not to mention less attenuation. Also they'll have to have a (much) lower value than the load, so in the range of \$\Omega\$, which means the C will turn out very large, and the resistor will have to be of great power to accomodate the losses, which also means it'll probably be wire type, which means it will have large parasitic inductance, which means you're better off using an inductor.

I assume the driving is done by a typical audio amplifier, thus voltage driven, thus low output impedance(?), so you could calculate your filter based on the data you have. Since the measured resistance is 3.3\$\Omega\$, you can deduce that the impedance is 4\$\Omega\$ (edited formula):

$$Z=4\Omega$$
$$L=\frac{Z\sqrt{2}}{2\pi f}, C=\frac{1}{2\pi f Z\sqrt{2}}$$

which will give you a Butterworth type of filter (also prefered for its flatness). If you need some cross-over, then Linkwitz-Riley will be preferred, but not mandatory, and then replace \$\sqrt{2}\$ with 2. Your choice(s) are, basically, any all-pole 2nd order filter is up for picking, not necessarily the best for the job, though.

The inductor will have to be made without a core (air core), to avoid nonlinearities, and with a wire that supports the maximum RMS current (it may turn out bulky), while the capacitor needs to be nonpolarized -- these tend to be more expensive so you can "cheat" by placing two polarized caps in series, with common + pin. You may also need an additional, small valued resistor (but higher power) to make it a lowpass RLC, for damping purposes, or for load matching, unless you get the LC values right, in case it's needed.