distortion with 12db/oct highpass

[bombardon73]

Some time ago I did build some Studio monitors for some friend of mine and lately he complained that on certain lowmid frequencies (between 400 and 800hz, especially some guitarsounds) he would hear some kind of distortion when mixing on high volume (more then 80 watts into the mid/high section)
It's more like a loud "click" or "tick".

I took home those speakers and came to the conclusion that it was the highpass of the MID speaker. It's a regular highpass with a crossover at aprox. 400hz.
L=1,4 mH and C=68uF, the speaker is 4ohm.
It tried several amps and other speakers to really make sure that it was the crossover.

Then I tried a 6db/oct highpass (only the capacitor), the whole problem was suddenly gone!

After that I tried other coil values, a higher value would only make the problem occur at higher volumes. Adding a resistor in series with the coil would do the same thing, but the problem is still there.
I also tried an LCR on the mid speaker to smooth out the impedance at the resonance frequencie, but without succes.

Are there other people who did have this problem, I think it's just the limitation of passive crossovers, right?
Maybe someone can tell me more about it.