AricAric,
While the D5000's only claim an impedance of 25Ohms, having owned them in the past, I can confirm that they do sound better when amped. The bass and especially the higher frequencies shine though when amped. Seems this amp is geared towards headphones with lower impedances. As per Amazon, the T1 puts out 1w@32Ohms and 400mw@120Ohms.
Hope this helps answer your question.