.8mv is still pretty low, that's not the problem. Also, I would use the 100 ohm setting not the 47,000 ohm. The Benz M series is at 24ohms the JC3 only has 2 settings for MC and the 47,000 ohm setting is too high for that cartridge. Also, the JC3 has a AC polarity switch that you can use to eliminate ground hum.
Regards,
Matt