I've only got direct experience with the dac1541 and ADI-2 Pro and DAC.
Generally I regard the RME to be a better deal than Soekris, if you think the features can help you. IMO the EQ can help out many headphones, especially bright stuff like the HD800. If you're used to THD in your chain, then the RME will sound too naked, even if it's the "right" sound. The headamp in the RME compares favorably to many of the dedicated stuff up to around 500$. Also the separate low-power IEM stage is virtually noiseless, which is good if you have something like a Campfire Andromeda or similar.
The Soekris does tick more "hi-fi" boxes. R2R, no-feedback discrete output stage... To me it's a sound that compliments the RME, as it's a tad more organic, mostly due to higher THD. I sometimes feel that the bass on the Soekris is weaker than on the RME, but I might just be imagining this. Both units have superb resolution and seemed virtually immune to source jitter, so I can't see much sense in clocking them externally.