You might try this:
Set the mains and the sub crossover at 80Hz. Set the receiver Front and play the 80Hz tone, it should go to the sub and the front.
Disconnect the sub (pull out the sub line in cable) - measure the signal level from the front.
Disconnect the front speakers (you have to disconnect the wires on the speakers, if you have banana plugs it's much easier), and reconnect the sub and play the same 80Hz through the sub.
Adjust the sub volume control so the signal level is the same as it was from the main speakers.