I've had my P10 with an Apheta 3 MC and EQ-500 for over 2 years. The EQ-500 has 2 controls that affect the overall gain. One control has 3 settings: MM, MC-HIGH, and MC-LOW. The other control is the gain control and it has 3 settings: 36dB, 38dB and 40dB.
The MC-LOW (for low output MC cartridges) will give you the highest gain. The MC-LOW setting will provide 63 dB of gain when the gain is set at 36dB. If you want the highest gain, you would set the gain at 40dB and use the MC-LOW setting for 67dB of gain. The MC-HIGH (for high output MC cartridges) setting only provides 55dB at a gain setting of 36dB and 59dB when the gain is set to 40dB. I'm getting these values straight off of page 11 in the owners manual.
I use the MC-LOW setting and 40dB of gain for a total gain of 67dB. I could set it lower at 63dB (gain set at 36dB) but I like to have my digital and analog volume close when switching between the two. If you use the MC-HIGH setting, I don't think you'll have enough gain at any of the 3 gain settings.
I just tried the setting MC-HIGH and confirmed that it's not the right setting for the Apheta 3. The sound was muted and less dynamic versus the MC LOW setting.