Yes, he is, that is why the recommended box size is smaller for more power, so the sub can stay within it's designed limits of movement.
Check the specs here:
Straight from the horses mouth
Lots of bad info around here. A larger box doesn't necessarily mean better sound people! It translates to better low end response with less power handling, and less upper end extension. This is exactly why a smaller box would sound better on the power he's running.
We are talking about a 12" woofer right? I never saw whether it was the 12 or the 15", I'm just assuming it's the 12 with a 1 cu ft box. If we are talking about the 15" woofer, than he surely needs a larger box //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif