anyone heard both of these?
I have 681, 668 (current) and 850 (past). The 850 have more detail, larger soundstage, better mid, look better, sound great without modding. The 681 have more bass and extend further down, very bright without modding but after mod, the high is much more natural with very good extension that beat 850's high.
The SRH850 is the HD668B. Same headphone, rebranded.
The HD681 is the little cousin.
My suggestion is the SRH850. Click the model number in my signature if you want more information. It's a very good headphone for it's cost.