The Slee is an exciting amp which pretty nice highs and great lows. It has good atmosphere for a portable, sounding sometimes a little warm, but it has a synergistic sound with the DT880, and while good (certainly better than many I have used), isn't perfect. But for pairing portable amps with the DT880, I might suggest looking for one with a bit of harmonic distortion.
The great thing about the DT880 600 is that it is quite easy to drive well, so getting bass and speed out of it isn't a problem. But its wide presentation and energetic highs can be somewhat problematic. Interestingly enough, the AMP3 Pro sounds very good with it.
I don't know your preferences, but among most portables, the Voyager is among my personal favourites for the DT880. But sometimes, the two can be too much like twins and not enough like partners. That said, some amps are simply too clinical: from bass to treble, they cut like a knife. The voyager doesn't do that.