Well, let's be real basic - no offense. When crossing a woofer to a tweeter, a crossover is used that filters out the lows going to the tweeter, and the highs going to the woofer. Metal woofers (and all woofers) have cone break-ups at frequencies higher than they are designed to produce. Since metal cones sound particularly harsh when fed any higher frequencies near their cone break-up frequency you want to make particularly sure that they get very little input above their crossover frequency. Now, you might say how does that happen if you have a crossover. Well, a crossover is not an absolute. It has a roll-off at the crossover frequency. Let's say a woofer cone breaks up at 10k. With a 1st order rolloff, and a crossover frequency of 2.5k, the woofer is only 12db down at 10k. This will be heard easily. Now, go to a 4th order rolloff, and the woofer is 48k down at 10k. This will likely not be heard as much.
Steep slopes are created by higher order electrical crossovers. With metal cones, you'd probably want at least a 4th order "acoustical" roll-off. This can generally only be created by higher order "electrical" crossovers. In general, the higher order the crossover, the more complex the crossover becomes in terms of component count. (This is not absolute, just a generalization. Depends on how many "shaping" filters are used.) The result is a rapid roll-off of higher frequencies in the case of a woofer.
With metal cones it is more imperative because they emit some real nasty break-up noise if they receive any signal at their cone break-up frequency. In many cases, the designer will also incorporate extra "filters" to eliminate specific frequency ranges above the crossover frequency at about the cone break-up frequency. These extra filters also add to the component count. Crossover design is a complex science/art. Folks will say there are simple textbook formulas to calculate these things. They are wrong in terms of actual application and it becomes more so when you factor in things like woofer breakup and tweeter resonances, not to mention things like baffle step for small boxes designed to be used well away from room boundaries.
Joseph Audio speakers are a good example of metal cones that can sound good although again it's a matter of taste. His designs use particularly steep filters. Some designers and listeners are of the opinion that such complexity detracts from the sound. In the end, it's a matter of personal preference.
