Initial thoughts are, as per the FDeck, -12dB/oct fixed HPF (it is possible to make the cut-off switchable but that adds to the enclosure size), -12dB/oct variable HPF, plus a -12dB/oct LPF.
It'd be unlikely to fit in a 1590A though. Need ganged pots for the variable HPF and LPF (for the uninitiated a ganged pot is 2 pots mounted on a single shaft).