Cs600 would be my choice - I think the analog stage there is going to be better than the headphone stage in the mytek dac, which if it's like many dacs with headphone stages it's just an afterthought for marketing and uses cheap op amps.
I'm going to guess that you are going to have to spend a fair amount - USD $700 - $1,000 - on a dedicated headphone amp to improve substantially on the one in the Leben.
I think you can use pretty much any level of headphone you want with the Leben. If you are REALLY into headphones and looking to spend much over $1,000, then it probably makes more sense to get a dedicated headphone amp to get the most out of your headphone investment. And that's assuming that you are going to be doing a lot of headphone listening.