Check your receiver manual if needed, but most will auto-set the volume for you when using internal generated tones to measure a 75dB reading on your SPL meter. Yes, 9/10 o'clock is way too loud for a EP500. I would think 7 o'clock is closer to where you need to be. Start there and if you end up being too far off in your receiver level controls, start over after adjusting the subs volume knob again.
Once all is balanced you can start experimenting over time with what sounds best to your ear regarding sub phase, crossover, and the trim settings. I would recommend you re-calibrate with your meter if you make any changes to these.
