Atom won't make much of difference in power. It's limited by supply rail. Surely it's not that the performance degrades with 600ohm load or anything. And more power for 600ohm won't make any difference in sound quality either.I have Beyer DT880 600ohms. I feel the DX3pro can drive my headphones to uncomfortably loud volumes on most of my music, and that's close to max volume in high gain mode. On quieter tracks, I'd love to have more room to go louder that the DX3 allows, but they still play loud enough for me to enjoy. I am looking to perhaps add an Atom to my setup for those times I want more power with those headphones, but am in no rush considering how loud it already plays.
If you want very high power into 600ohm, you either want a discrete design or a balanced output. The former can have much higher supply rail that allows much higher voltage swing hence needed for 600ohm load. The latter, will give 6db extra headroom , 2 times the level, 4 times the power.