The subs have a 4 Ohm impedance

But SVC or DVC?

If DVC you can get a final single ohm load of 4 ohm, by wiring them in series/parallell to the bridged amp. This will give you the max power the amp is capable of (each sub will get the same power as the amp delivers on a single channel into a 2 ohm load)

If they're SVC, you´ll have to wire them in series, for a 8 ohm load bridged. You´ll get the same power as if you wired each sub to its own channel (ie what the amp produces per channel into 4 ohm)

The benefit of bridging is that both subs play the exact same signal, not one playing the left signal and the other the right signal. (nowadays probably not that much of a problem as the bass typically is present in both channels, but on older recordings that may not be the case)

Common active filters typically have 12/18/24 dB/octave.

No real problems with simple cap/inductor filters - for killing bass to front speakers they´re commercially sold as "bass blockers"

/Michael