not sure how its the upper limits of the sub if he's not even playing below 35hz. I've seen people run 3k to a pair of sa 15s wired to 1 ohm and they didnt bottom unless they played well below port tuning with subsonic off with a moderate clip.Pull the sub and free air it a bit to see if you hear any ticks as you take up the power
I don't think it's the h/u
Many many people have used those eonon units with great results
You probably are near the upper limits of the sub