No its not bad to under power a sub, but it is real bad if you dont know what you are doing!!! Its called user error.
As for power on a XL i would be looking at atleast a 2000+ watt amp for that sub. In real world you wont get anywhere near 1500 watts from that amp daily at rated impedance. I tested one of those amps once and was very disappointed with the results.
I play lower, louder, and longer thanks to