I'll just copy/paste here a response I gave in a similar thread:
They're very similar.
The Samson SR850 is essentially a Superlux HD681B, rebranded.
Some people assumed they're identical to the HD668B (cup design shared some more similarities than with HD681 line). However they are not.
Driver coils & magnets and impedance are different, 32 ohm for the SR's and 56ohm for the HD668B.
The Samsons, like the HD681's, are a bit louder compared to HD668B for the same volume settings.
HD668B scale up a bit better with amplification, while the 681 and SR headphones don't improve as much.
Sound signature wise, I find the Samsons a bit drier and slightly more edgy.
HD668B is a bit warmer in the midbass and treble is slightly different.
Both require EQ for the treble. HD668B is more timbre accurate and natural after EQ than both SR and HD681, especially on acoustic instruments.
The HD668B don't do well with EQ when adding bass. They lose some definition and sound a bit too boomy.
HD681 and Samson does better and can be transformed more readily into a fun sounding headphone, than HD668B. For modern music I'd give a clear edge to the former.
With velour pads I find the SR a touch more comfy, because of the pleather headband, instead of wings. But HD668B improve more with velours in sound, than SR's do (they sound even drier with velours vs. pleather).
Overall the differences are subtle and essentially taste dependent.
If you're a jazz&classical fan, the HD668B will have a slight edge. If you're more into modern music, the SR might be a better fit.
Both require EQ or some form of modding to sound right and both desperately need velours for long term comfort.