Youre giving an 800wrms rated sub potentially 450watts. Yes, thats underpowering. HOWEVER- many people underpower their systems. Even if you threw a Sundown 3500 on it, you still may underpower it because of impedance rise, voltage drop, heat, etc...
BUT, youll be JUST FINE with that amp if you set the gain knob correctly so there isnt clipping. "Underpowering" a sub will not damage it at all as long as the amp is putting out a clean signal and the box is feasible. Just stick with a bigger enclosure if itll fit properly.