Hi.
I see many XOs use air core inductors; many use steel laminate (steel core?) inductors.
Is one inductor better than the other? If one is clearly inferior, is it used because it is much cheaper?
My education continues....
An air-coil can be best if you are willing to spend the extra cash and have the room in the speaker for some larger components. For woofer designs you want to keep the component resistance as low as possible. In order to keep resistance low in an air coil, you have use a larger gauge wire. When you have low crossover points the amount of wire needed can get huge and with today's copper prices, this means much higher cost. Using steel lamination allows for the use of less wire and it will keep the resistance down without having to use large gauge wire. Much less money involved but the use of a steel/ferrite/whatever core brings about compromises in terms of potentially higher distortion effects under higher power levels.
Air-core coils are preferred but the cost hit can be high for a what will likely be an incremental benefit in terms of sound. It's really about balancing the overall design.