Finally! A question on here I can answer! I literally own only two stacks: the e30/l30 stack and the atom stack. And I bought a set of hd6xx like two weeks ago. (I was born for this moment!)
Simple answer is the right one, you can’t go wrong. They both sound good and are fun to listen to on hd6xx. I’ve landed on the e30/l30 stack as my daily driver, with hd6xx on the headphone stand next to them. And every day I have a great time listening.
Atom stack is great, and without a lot of side by side comparison I couldn’t tell the difference. But when I started mixing different dac and amp, I found that I liked the sound (and features) of the e30 dac over the atom dac. If you are looking for longer term scalability, the e30 will pair nicely with many amps (imo).
For a while I ran e30+atom amp (before l30 was available). Really nice pair. I ended up getting the l30 mostly to have a matching stack. But the l30 does have more power, which I think I can hear in my harder to drive t50 mods. But to be honest, they sound very similar.
Hope you enjoy whatever you get!