Thank you very much for your prompt response, Mr Elias. Its great to see you're still active here and so knowledgeable about your product, I wish more manufacturers would aspire to such support. You're a gentleman and a scholar
I wont pretend to fully understand the link you provided listing the technical reasoning against balanced operation, however this seems to fly in the face of conventional wisdom on these boards. Its been drilled into us like a religious sermon that balanced operation will always exceed unbalanced operation in terms of extrating the last ounces of sound quality from a headphone, especially with the Sennheisers.
The reason touted is normally the elimination of crosstalk and the doubled voltage swing provided to each driver - the Senns are reputedly very voltage-swing dependant, and subjective listening tests seem to have verified this.
I'm not disputing your information, however i'm just curious as to why oppinions are differing so much on this. Even if the balanced line-out stage of the DAC1 USB was somewhat inferior to the headphone amp for the purpose of driving headphones, I would have wagered that a balanced HD650 would still far outperform an unbalanced HD650 with the DAC1 USB as source/amp.