The dss has a bit more power and less hiss than the mixamp. However it may limit your chat options.
Dss2 does not use true Dolby headphone. I have not heard it myself but I've read it is not as good as the original dss.
No, it's not a horizontal clamping issue. These were an AWD purchase and the clamp force is pretty light, so perhaps whoever had them first stretched the headband. I believe that my issue stems from weight distribution. The 400i are only 10g lighter than the X2 but in use they feel so much lighter because of how they hug the head.