the actual DB output has only a modest amount to do with it. I've seen people overheat amps without it being 'loud'. i;ve done it myself.
I overdrove the shit out of my old fosgate 125.2 between the bass eq and the level adjsutment on the xover. I could cook on it, but it still wasn;t super loud. In retrospect i should have gotten a 250.2 but I chose it because i had little room for anything larger and i made it work with ducted fan cooling.
in your case you said it is a 500 watt JL. which 500 watt JL?
is it a slash series?
what is the impedance of your subs as they are connected to the amp?
what is your amp sensitivity like? maximum?
headunit bass is at 0, or maxed out?
15 or 16 on the head unit means absolutely squat.
you could be overdriving the hell out of the amp at 1/3 volume if the gain was set up too high.
I still think you have way to much eq or gain somewhere.
try using it at 12 or 13 on the head, I bet the amp runs a LOT cooler. if so you have too much total gain.