Let's make this easy, for a ~4 ohm impedence at or around 80hz, you will need the following:
6db - 500uF cap
12db - 350uF cap, 11mH shunt coil
For ~8 ohm impedence:
6db - 250uF cap
12db - 175uF cap, 22mH shunt coil
These values will get you in the ballpark and most likely be just fine and fine tuning the values in relation to the exact impedence at 80hz will yield better results but in most likelyhood, your room acoustics will swamp any differences an exact impedence match would reveal.
Use ~100 volt non-polorized caps in the main feed and build an external crossover box...
Cheers
6db - 500uF cap
12db - 350uF cap, 11mH shunt coil
For ~8 ohm impedence:
6db - 250uF cap
12db - 175uF cap, 22mH shunt coil
These values will get you in the ballpark and most likely be just fine and fine tuning the values in relation to the exact impedence at 80hz will yield better results but in most likelyhood, your room acoustics will swamp any differences an exact impedence match would reveal.
Use ~100 volt non-polorized caps in the main feed and build an external crossover box...
Cheers