A series cap by itself will not work as a crossover with a piezo. Look up the piezo crossover posts from djk on the subject. By coincidence, I am wiring up a crossover for a dual piezo today based upon what he has written. The piezo is pure capacitance, so a capacitor in front of it reduces it's output but does not cut the lower frequencies. This is what I am doing: Put a 75 ohm (give or take) resistor in series with the piezo. Before the 75 ohm resistor, put a resistor across the + and - terminals, or better, a resistor and small inductor. (I am using 7 ohms and .1 mh in series) Then before that, put your crossover. I am going to use a 12db/octave crossover at about 10k for my setup. The crossover "sees" the resistance across the terminals, but is shielded from the piezo by the 75 ohm resistor. At least that is my understanding. Maybe somebody with more knowledge will respond.