Passive bell shaped EQ for headphones

Hi, I would like to make a passive bell shaped EQ circuit with a constant value for my German Maestro GMP 8.35 d. I wish to tame the highs:

Can I do it passively as an adapter between the amp and headphones?
Where can I find a calculator?
My values at the moment (after tweaking with VST) are:
1.25 oct

Thanks for your help
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