> I have both iron and ferrite cores in toroidal form
Sorry, I forgot that there is a powder-core called "iron". It is small bits of iron in a non-conducting binder. This is "not as good" as solid strips of iron when you want audio frequencies.
And yet there are some pretty exotic cores in audio today. I doubt they are radio-cores, they are probably bleeding-edge new-product.
> wait a sec, L and C in series is a resonant bandpass, isnt it :/ haha
Well, sure. But you always have resistance too. If all the resistances and reactances are similar at resonance, the Q is about 1 and the response is flat to a point and then falls 12dB/octave. That can be a perfectly acceptable response shape. You can fudge things so Q is about 0.7 and get a Butterworth maximally-flat response, go Q=0.5 for a more Bessell linear-phase response, or run Q=1 or 1.4 to get a several-dB bump that cancels other fall-offs in the system.
But you sure can get in trouble if you forget this. A classic case is a capacitor and transformer off a low-impedance source (op-amp) and no or hi-Z load. You can end up with a huge 2Hz resonance, which you may not notice in testing, except all the music sounds unsteady from all the subsonic noise and garbage straining the system.