In general it's better to start a new thread (you can always include a link to the old one) rather than continuing a very old one... I thought about splitting this into a new thread, but I'll leave this one for now.
Anyway, the behaviour you describe sounds very odd - unless you have frequency dependent (i.e. where the component value varies with frequency) components in your circuit, as I pointed out towards the top of this thread.
What is the make up of your passive filter? How is it represented?