I wouldn't feel comfortable recommending the use of a DACport LX without an amp for anything other than fairly sensitive IEMs. There's just not enough power to drive your AKGs.
Even though the volume might seem sufficient at first listen, I found the LX to be really bright with my LCD-2 and even more so with my Sennheiser HD280. The reason they sound bright is because there's just not enough power to drive the lower mids and bass, but plenty enough for the higher frequencies.
The DACport LX sounded great going directly into my SE530s - it was incredibly transparent, to say the least. The ultimate transparent amp is no amp at all. Problem is, you need really efficient transducers. That said, it's still far from ideal because you have to control the volume with your software - I found it really difficult to get the volume low enough for some tracks I played.
The line level output of a DAC is very loud, but there's just not much power behind it. I'm guessing here, but I think it's a matter of having lots of voltage but not much amperage, thus not enough Watts to drive anything that's inefficient.
In summary - You might enjoy the LX with your IEMs, but not with either of the AKGs.
Mike