is it possible to have a subtractor circuit with a low pass filter in it? My thought is that I would have the possibility of doing that if I add a capacitor in parallel with R3. Is it actually possible? If not, whats the best way of doing it?
Yes. I have done this before. You put two capacitors in; one across R3 and one across R4. But remember the impedance at the noninverting input is not the same as the inverting. I will leave it to you to determine what they are, especially as you have not given any values.