Some of the Martin Logan speakers do not have the ability to bi-pass the internal crossover. In that case bi-amping is still a benefit--but not as great as when you can get rid of all those components within the speaker. You definitely want to go active, which means you do need to buy the external (active) crossover. If these are for Monolith 3s then you need their crossover due to some phase variations they have built in there to make the woofer/tweeter a bit better in terms of seeming the two.
