If you don't have a multimeter you can get one for $6 at harbor freight that will do what you need, or where ever you can get the most basic one is fine.
Turn every eq or boost setting to flat. Then check your AC voltage like this.
Then you can at least see how many watts you are sending to that speaker without changing anything. Not sure how it was set, but I would expect any JL amp is going to be pretty close to rated power. If you're only sending 500w, then you are missing a lot. If you are sending 600w, then you are missing about 2-3 more on the volume knob (which is noticeable).
I just flat out wouldn't spend money how you say on that amp, just to be barely capable of just this sub. You can dial any amp down. If there is ANY CHANCE AT ALL, that you want to add more subs, or different subs, this is the time to buy that bigger amp instead of that one. Two of those $1K subs would be at 1.5ohm, and anything that is rated at 1 ohm can push at 1.5 ohms. Three of those subs is 1 ohm, and finding an amp that can do 2250w-even up to 3,000 (for 3) @ 1ohm is not hard to find, and leaves you some room for down the road.
I like JL amps, it's good stuff, but what do you need super clean power for on a sub? That amp is rated .03% THD at rated power. That's great and all, but you're not going to hear the difference between .03 and .5% THD when the bass is hitting. That THD rating, is what you are paying about twice as much for as an amp with a little more that will do the exact same thing, or will even be big enough for two of those.
I always recommend this link to see if someone can find a deal:
WoofersEtc.com - The Online Car Audio Superstore, the Largest Selection at the Lowest Prices!
www.woofersetc.com
IMO, the next step is you figuring out what is your end goal. How much would you like to really get out of it when you're done. It sux buying 5 amps that are just slightly bigger than the last. When you could have just bought a big one in the first place, and only needed to adjust the gain. Do you want more bass if you could? There's a little left in what you have, but if you want a lot, then don't waste your money on baby steps.