This may not be simpler but it is just another way inspired by looking at the Electro -Voice academy award winning 642 microphone internal 2 position filter.
The filter is connected to the 50Ω output tap of the mic.
So why not when winding one's own ring core input transformer not include a very few turn (low impedance) winding tap to the primary or secondary. Or even a few turns of a third winding.
So say a 10Ω local output impedance is created.
Then making an inductor cutoff filter would be very easy. A very small inductance is needed for given cutoff. Only s few turns. This filter will be connected in parallel or shunt to the low impedance winding.
And a resistance can be added in series to this external coil for a variable low cut or high pass slope. See improved Pleiades filters.
References:
Electro-Voice 642 engineering data from the Coutant site.
http://www.coutant.org/data/642.pdf
Improved Pleiades filters - euroelectron
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