Just a Suggestion

You know the right values for the 12dB/octave at 6000 Hz

For the 18dB/octave C1 = 5.6 microfarad and C2 should be 16 microfarad
The coil L1 = 0.1mH. This will be a 5000Hz crossover.

 

Its a small ceramic capacitor. Sometimes when we use cheaper quality caps there can be some leakage. Adding a series capacitors "removes" the effect of leakage inductance. Choosing better quality components don't need fixes like this. Also smaller cap values don't have this problem. So in that circuit, it was completely not necessary.

Another way to prevent a problem is to use two 99uf in parallel instead of a single 199uf. Small value caps have much less problems than large values.