Re: please help to enlighten me on some passive filter quest
Not sure exactly what you mean, but yeah, ground planes make a huge difference. For one thing, you need uniform distribution of ground currents, otherwise you end up with parasitic inductances that throw your bandwidth and center frequency off. A good example is a lumped filter where a resonating L-C pair are used. The LC pair had better be mounted right next to each other, beacuse the circulating current will be high and if they have to send that current thru a long and winding ground plane pattern, then you will have unpredictable consequences.
Also, since stray capacitance can load down any filter, even distributed ones, how close the ground plane is makes a difference. Sometimes, if you have an odd ground plane geometry, like a close bottom ground plane but a far top ground plane, you can excite some odd dispersion effects.