Barry, do you mean that you want to use a single amp to power just the woofer (with another for the M/T,) or do you mean a single amp to passively power the whole speaker?
A line level filter will use resistors and caps, no coils (although it could use coils, but not necessary.) Like all analog filters, Line level filters are sensitive to their load impedance, so it helps to use a current stage amplifier to make sure the filter parts don't rob highs or lows due to insufficient drive. They are called active when they are powered by an opamp or some active amp stage. But pure passive line level filters are possible in some cases. These would just insert into the line before the woofer amp. If you want any kind of boosting of low freqs then a passive filter is not possible.
A Linkwitz Transform is a type of filter that boosts bass in sealed boxes. It must be active, that is use a buffer like an opamp to power the filter.
Any kind of analog filter will cause phase error, so they can't be applied to the same cone with two amps, no matter whether passive or active. The phase difference will cause the amps to overheat, protect, or break the VC former from the tug of war.
A linear phase digital crossover could concievably accomplish what you are imagining, because that type of filter does not add phase error to the filtered signal, just amplitude attenuation and that's all. But it has a half second latency so the whole spekaer must be active crossover, and they are pretty expensive because it requires huge CPU power to calculate the filter values for each sample individually.
OK now you got me rambling... DANGIT