In my experience, the Atom could play louder and cleaner than the Spark. Don't be afraid to push the Atom's volume past 12 o'clock in low gain mode, coz that's the optimal region when using high impedance HP such as my HD6XX.
I prefer using D50 with the Atom (opposed to my D10 and Modi3) because the D50 provides different PCM FIR modes. I use Mode 4 when listening to highly compressed Pop music to minimize harshness and tame the edgy sound. My Schiit Loki (EQ) cannot accomplish that feat.
Among the 3 Dacs, the Modi3 generates the most analytical sound (but I could also make the D50 sound similar using one of the modes). For that reason, I use the Modi3 on my 2 ch stereo system to maximize the performance of the Raal tweeters on my Philharmonic Audio BMR speakers (powered by NAD 375 BEE), but thats for another topic and forum.