Fundamentally, all you need is a capacitor in series. Most preamps or amps will have a specified input impedance, and if designed properly, it will be resistive in nature. The standard RC network equation f=1/(2piRC) can then use this input impedance as R, to find the correct value for C. Adding a resistor in series (as in your first link) will add to the overall R by simple arithmetic addition. Adding a resistor in parallel (as in your second link) will decrease the overall R according to the given equation, which gives the net resistance of two paralleled resistors. Note that the series resistor will decrease the input sensitivity of your preamp or amp, while the parallel resistor will not change it. Neither resistor is strictly necessary, but neither will cause any harm either if they are good quality and don't cause a major deviation in the input impedance. This all assumes that the input impedance is resistive to begin with. If it isn't, then a passive filter is not the way to go. Hope this helps.
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