I just thought I'd post this to follow up on your questions regarding LC filters. Shown is a low pass filter with a -6dB knee @ 17KHz. The source impedance of VG is 600Ω and RL = 600Ω. As far as resonance is concerned, this is a low Q circuit so the curve looks virtually flat until cut off freq. However, if the load (RL) is lifted a visible resonance peak will occur at about 18KHz but passive filters are designed around a known source Z and load Z.
Chris