I owned Icon up until few days ago, and own uDac also.
uDac is superior to Icon's built-in dac. Soundstage that uDac makes is slightly deeper... "blacker" background also...
uDac's headphone-out is louder (more powerful?) then Icon's, but I think Icon's headphone gives better sound.
I also have Travagans Red, little amp I use to drive my desktop speakers. I prefer Travagans' to Icon when listening to speakers (Travagans' soundstage is much deeper), but Travagans' headphones-out was bit harsh compared to Icon's.
Icon Amp is said to have slightly warmer sound compared to Icon. In few days I'll be getting my Icon Amp, so I'll post my impressions.