Well YA! If you are underpowering your sub your not going to get your full potential out of it. Plus it always is going to leave you wanting more. Which means you will want to turn up the volume more and more and you will melt your voice coil with lots of distortion.
depends how underrated it is, for example, some 1000w rms subs, if you give them 900w, they might reach full excursion, but if you're undepowering with like 600w rms, you're going to notice some difference