you can get an eq with the front, rear and sub RCA outs , from a single RCA input and control your sub with that. the options vary from eq to eq.
if you go with the crossover, again its dependent from xover to xover. you can get one with sub equalization, Q control, cutoffs and boost, remote bass gain, blah blah blah.
or like andy1234 said you can use a decent HU with subwoofer output and internal xover, Q and gain control to get what you want.
your orion 1200d already has:
LP 30 - 250hz @12/24 db octave with optional Remote level control
and your orion 4004 has Crossover range/scope HP/LP @ 100hz
what exactly are you trying to do?