Assuming your C10R speaker is 8 ohm, 8.1 volts would be about right, if it’s a 4 ohm speaker, 8.1 volts would be high, maybe too high. Around 5 volts would be more appropriate. The “1/3 of the speaker power rating”
is too simple an approach and might explain why some have had problems trying to break-in speakers using it (IMO). I bet the magnet would get pretty warm running 8 volts in a 4 ohm speaker and that leads to the voice coil coming apart.