One can easily calculate the attenuation of a voltage divider formed with resistors. Where things become a little bit tricky is when the reactive impedance of the driver and of the crossover components is figured in. You can actually get peaking from reactive forces that is greater than attenuation provided by the resistors in the L-pad. Such peaking can form a narrow peak, or it can form a very wide band, having influence wider than an octave. This is dependent on the ratios of resistance to reactance, and so the easiest and best way to analyze the whole picture is with a circuit modeling tool like Spice.