I was assuming since you mentioned you calibrated them all to 75, that you ran Audyssey (or equivalent) on them and saved to a preset? Or did you just mean used the RS meter to set the levels to 75?
If you haven't, that would be my first thought, see how close Audyssey can get you for the mains, since its easy / one button approach. Or would Audyssey not have sufficient EQ horsepower to address the FR? I use MCACC, not Audyssey, MCACC only has set frequencies on EQ, not sure if Audyssey's are parametric - if not, guess that may not help.
(sorry, forgot what receiver / processor you're using, not sure if Audyssey applies)
Otherwise, for REW and the separate parametric EQs, then you'd take a measurement, push the EQ button at the top to bring up the EQ panel; drop down "Equalizer", set to "Generic"; drop down "Target Settings", select "Speaker Type = Full Range", adjust crossover settings as needed (how you're crossing them over to the sub); click "Set Target Level"; drop down "Filter Tasks", set the "Match Range" to whatever frequency you want to try to adjust (like 80-12,000 say), set individual boost to somewhere between 0 and 3 dB, overall max boost somewhere between 0 and 6 (YMMV), flatness target 3 dB, then click "Match Response To Target", which sets the filters. Then click the "EQ Filters" button at the top to see what filters it set up - and you can manually add filters and see the predicted effect if you aren't satisfied with what REQ came up with automagically. You can then manually enter those PEQ settings into your PEQ for that channel.
Repeat for each channel.
. It's as simple as REW with the rat shack meter, but accurate.
