Thanks so much for your response, and I apologize for my inadvertent ambiguity - I don't know where the 'and out' came from, and I understand your confusion. It only goes to Main L/R of the headphone amp, and I have edited my post for clarity, and also I have a pic here that basically shows the config.
I actually also have the exact cans you're referring to, and they sound the same as well. very confusing. Any additional thoughts would be very welcome, and again, apologies for the confusion.