Okay I need to think if it's worth it.
My original plan was to use my AV amp and take the pre outs and split them with simple RCA splitters, one going to my mains getting fed a full range signal and the other going to my amp for the flanking subs, the flanking sub amp has dsp so I was going to just use that to put the low pass on the flanking subs and any eq/delay needed.
Now it means I need to get a dsp module ( probably mini dsp) to go between my mains and AV amp, because if I high pass the mains the flanking subs will also be high passed.