100mW min will be ample unless you want to smoke your eardrums. They can handle way more power than you could tolerate. I have two 650mW class A amps and neither has any issue with strain at any volume I've attempted.
So this unit certainly has plenty of power, but many options exist. I have an audio-GD 12, entry level unit, and frankly though it has tons of power, it doesn't sound as good as my DACPort with the T50.