I think your expectations are just too high. The HD600 is a terrific headphone but the law of diminishing returns applies to all things audio. Properly driven (yes, the O2/ODAC is a great choice for the HD600) they will sound better than unamped, but it wont be dramatic. You're likely to be let down if you get your expectations too high again. Improvements above your a900x are going to be relatively subtle and incremental.
Keep your expectations realistic and you will certainly enjoy and appreciate your Sennheisers very much. Just dont expect any revelations.