I wouldn't expect it to work all that well for any situation that overlays existing passive crossovers. Seems like you'd get a hole/dip in response at some point regardless where it's set.
For overlaying existing passives, separate high pass and low pass is the ticket. Active HPF would be set about an octave below the passive HPF and active LPF about an octave above the passive LPF. I'm sure there are some exceptions but that's the generality of it.
I know exactly what you're saying...but I can't find any active HPF or active LPF.
Do you know a source for these ?