Nothing really. When you add filters, the overall level decreases. So, when you remeasure after modifying filters, its best to go through the whole REW calibration with Check levels etc. It only takes a second, and this brings everything back to the ~75dB target.
Good job, BTW.
Now add the mains and (recalibrate exactly as before when you were just measuring the sub) and see what the crossover transition is like...
brucek