Hello,
Alright this is not passive, but there is the Wien Bridge Parametric Equaliser.
I’ve used this in conjunction with a Baxandall tone control circuit to give HF and LF EQ centred on 'say' 500Hz and the Parametric Equaliser is used to give a ’sweepable’ middle control. With the right values you can get a middle control that sweeps between a couple of hundred Hz and a couple of KHz.
Link to some info on the Elliott Sound Product site
https://sound-au.com/project150.htm
Regards
Terry