I made a simple passive crossover for my Hi-Fi sub (100Hz, 12dB rolloff) and it worked well enough. I tried it on my bass rig out of curiosity and it didn't sound crash hot. It actually distorted a little bit. Turns out the Iron core inductors I used aren't suitable for crossovers and I should have used air core. My problem is this, air core inductors seem to have very small mH values, so I'll need heaps of them to get the desired inductance. Where can I get large Air core inductors? At present the biggest I can get is only 3mH.