Would there be any interest in a small PCB that has a configurable filter on it specially to 'correct' headphones ?
I am thinking of making 2 PCB's for my own but to get an interesting price (around 10 Euro or lower for a bare board) I need to have at least 10 PCB's made.

It will be for my modified T50RP and T40RPmkII coming soon (I hope)
Of course this PCB could be used for other headphones that need a nudge as well as the filter(s) can be set to make all sorts of filters.

It will have one gentle slope (6dB/oct) filter that can either be filtering lows upwards or downwards or highs upwards or downwards.
The frequency point and amount of dB's can be set to desired levels and points.
There will also be a second notch filter section that can either lift or filter a frequency.
The centre frequency can be configured to be anywhere between 20Hz and 20kHz (below and above seems pointless  ).
One will be able to set the amount of filtering (dB's) with a small pot and the bandwidth/Q can be configured to match the needed response.

Once set the filter can 'equalize' the FR but cannot address ringing (in fact the notch can introduce a small amount depending on settings).
It can thus only be used for 1 headphone.
I will make a tutorial for it and will assist if pirates need a certain filter 'calculated'.

It must be fed with a +/- symmetrical DC voltage which must be provided for externally.
just a few mA is needed and voltages between 12V and 15V will be perfect.
depending on the needed correction, gain and source voltage a lower power supply voltage could be used.
It can be drawn from an amplifier itself or via an external PS.

So in essence a tone control that can be set to boost or filter gradually from a specific point + a notch filter that can boost or filter a specific area.

The price will depend on the amount of PCB's and if I can get a good layout on 2 layers, otherwise it will be 4 layers.