X5III will be way more than sufficient. As a general rule, you don't want to have 'just enough' power, as that means your source is always near its limit. You will always want a bit more, but not so much that you will never going to use it. Also, unless you are planning to use just one pair of headphone forever, the smarter thing to do is to have a source with just a bit more output power in case your next headphone isn't as easy to drive as your current headphone. Of course, with such an high output as X5III, there is actually only a handful of headphone that it can't drive well.
Have you tried driving a HD650 and HD800 straight from the X5iii without amping? Does it perform well?