A square wave has 2x the power of a sine wave of the same amplitude, with the extra energy taking the form of harmonics (specifically, the harmonics are all odd harmonics, with each harmonic having the amplitude 1/n, where n is the number of the harmonic). What this means in practice is that if you drive an amplifier into hard clipping (so that you are getting square waves out), the power delivered to the loudspeakers may be up to 2x the rated power of the amplifier. Now, as for the high drivers, if we consider the case of a 100W amplifier driving an 800Hz into hard clipping (so it becomes a square wave), the high driver is going to see a bit less than 100W (and your low driver will also see around 100W). Now, high drivers aren't generally rated for the same full power as the low driver, so that's generally a bad situation. But in practice, the crossover will be dissiapting a good chunk of that power (through the frequency dividing function, and through the pad that's likely present to get the sensitivity of the high driver in line with that of the low driver), so that will help matters. But you still run the risk of frying equipment. OTOH, if you aren't clipping your amplifier, you have nothing to worry about.