What are your room dimensions, and where is your listening position ? I noticed you said the sub is at 65% -- that seems pretty high at first glance. Wondering if there is some kind of response dip in the room forcing you to turn the volume up higher than normal to compensate, giving you a boomy result...

You mentioned another sub -- have you tried putting it in exactly the same spot as the EP500 ?

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8