nothing would happen to them you just wont get the full potential from the subs. But yet again sum amps like fostgate might go on protect if the subs drowned the amps power or if its over powering them witch sucks.
Under powering sub does nothing to your subs. You can actually get a lanzar 2000 watts rms on ebay for around that price and I have heard they are perty good amps.
Keep in mind that your subs, when wired together, will be a 2-ohm load (assuming you're using two subs).
That means the first amp you linked will produce 300 watts RMS, not 600 watts. Each sub will get 150 watts.
That said, using an amp with less than the subs' rated power won't harm anything. They just won't play quite as loudly as if you had the full power available.