Ericsson@Ericsson Good question. I don't have the HRT yet but I have both O2 amp and Odac. The O2+ODac as a combo needs to be powered by an AC adapter and is also larger. The amp section of the O2 is much more powerful than this though. HRT states 150mW but doesn't say what load. I suspect it is into something low, like 16-30 ohms. By that, the O2 amp is about 10 times more powerful. The ODAC alone costs more than this too. Personally, I'm getting this cause I'm lazy to lug around an amp+dac.<br /><br />@Champion This would be perfect for that sort of headphone. Low to Mid range headphones with lower impedance.