Honestly, bulbs work better for tweeter protection than any fuse or breaker I've seen, as they compress the signal rather than fusing. The bulbs only blow if the signal is much higher than expected or if the load is open or shorted. I suggest that you get out a meter and start quantifying things. Check the voltage level of the amplifier when the bulbs blow. Those bulbs don't cost much, so it isn't any trouble to make a few tests that push them to the point of destruction. Also, check the impedance across the crossover's tweeter output. Check the voltage across the tweeter output. Once you know more about what's happening, you can make a more informed decision how to correct it.