Get a bigger amp if you can. IIRC the rms rating on the UL is 2500 watts, and a lil headroom would be nice too.
That's the downside to these uber woofers guys, they want to be sealed and have a ton of power thrown at them. I imagine they work okay with less power, but then you wont be getting near the full potential of out the thing you paid alot of money to have outshadow other drivers, drivers that would likely work better with that smaller amount of power.
Hoffman's Iron Law confirms what I suspect about the driver: low efficiency to allow good low-end while utilizing a small sealed enclosure. If you want your UL's to work to their fullest, use this info accordingly.