The end of the excursion of a speaker is the point at which a speaker, which goes back and forth to produce sound, can go no further. You can hear a thud or whack when you slam a speaker into itself by feeding it too much power. Sometimes, this does no noticeable damage, but if you push it too much, it can be ruined. Take a look at:
http://electronics.howstuffworks.com/speaker.htm
More specifically, look at the diagram at:
http://electronics.howstuffworks.com/speaker3.htm
You will need to click on it to "dissect" it, and look at the voice coil, and think about what happens if it goes too far toward the magnet in the back. You can have another bad problem by having it come forward too much.
You can see it in movement at:
http://electronics.howstuffworks.com/speaker5.htm
Your Yamaha is capable of delivering a full 100 watts into one channel continuously, and more for a brief moment, so it is likely that it can push your speakers to such a point.
If you set the crossover to a higher point, it is likely that you can play it a bit louder, though it really depends on the fequency content of what you are playing. The downside to doing that is that you are more likely to notice where your subwoofer is from the sound, because as you go from deep bass to higher frequencies, your ability to localize the sound (i.e., tell what direction it is coming from) increases. Still, it is worth a try if you really want to play things louder. But be careful, as the next time you hear such a sound as you describe from your speaker may be the last time it works.
If you really like things loud, you need to get speakers that are capable of making loud sounds. However, if you value your ability to hear, you will want to keep it within reason anyway, as loud sounds lead to deafness. You can search the internet for information on such things. Many people damage their hearing with their audio equipment.
