If your subs rms is 400 and the amps rms is 350, then you should be fine. I don't think it's a good idea to always run your subs at the nominal power ratings anyways. And with only 50 watts difference, your not really underpowering your sub enough to cause any damage.
Actually underpowering a sub can cause damage. The amp desn't have enough power that the sub is demanding, and tries to put out more than it can, causing clipping. You can live with very minimul clipping, but severe clipping can destory things.
Anonymous is correct, you can't damage a sub by underpowering. And Joe is right, you destroy the sub by the CLIPPING, not the UNDERPOWERING. But if you want to underpower a sub, you need to be careful with it, and not try to get an extra 100W out of your amp, which it can't give you. That's when you're going to get your clipping and destroy your sub. If you know you won't be able to discipline yourself to not blast your stereo, than you probably shouldn't try it.
It's basically when you try to push too much power out of your amplifier, and the amplifier isn't able to produce that much power. At a certain point, it will begin sending damaging voltage to the subwoofer coil instead of good voltage. There is a chance you will begin to hear the sub distort, although you may not. Even though there isn't distortion, you may still be clipping the amp.
Nice link Jake. I know that underpowering a sub alone will not destroy the subwoofer, that would be kinda stupid. The point I was really trying to make was that HE WOULD BE ABLE To turn his volume up high, and not have to worry about his amp clipping. Thanks for politely correcting me though jake.