Bitcoin is in a big bull run. Almost all alts are taking a hit. I would advise hold your AMP till 2017.
It hasn't dropped more than 15 percent because the BTC bull run ( there wasn't a significant bull run since yesterday, other alts didn't drop down 15 percent ), a AMP whale put up a 200 k AMP sell wall and traders started to bend over and the price dropped fast.
Without manipulation the price should be -5%