Technically, 80mW should be sufficient because the T1s should only require 63.1mW to produce 120dB (which is more than enough to cause hearing loss), however my experience with the T1s has been that while they don't need huge amounts of maximum power, they seem to benefit from a reasonable amount of power and this may be to do with the voltage swing and/or available current required as marshallmole said. Power output can be split between different ratios of voltage and current as I understand it so perhaps the soundcard is producing sufficient voltage, but not current or vice versa - this is getting beyond my technical knowledge, but I know what I've heard and the TH-900s seem to thrive on less powerful amps while the T1s need a bit more juice in my experience.
While I don't disagree with this in theory, I must have auditioned about a dozen different portable amps before taking the plunge with a desktop amp - despite some decent output power stats, not one of them actually had enough juice to drive the T1s. I'd have the volume maxed out and it would still sound too quiet. Then again maybe I've just killed my ears with years of punishing techno :D