That is it. The resistor 'bends the knee' of the lowpass curve (and reduces the boost just under fc). The higher the value, the more you will end up with a first order crossover (along with the phase shift involved of course). If you're able to measure, try anything between 1 and -say- 4Ω7. If not, start with 2Ω7 and increase if necessary. Calculating with imaginary drivers can be done in Boxsim, Xsim or any other sim app.
You should consider to take out the speaker terminals as it allows you to play freely. A crossover out of the box is already a good thing 😱😛