I can't imagine a single person here would be happy or content buying a 660S2 from Amazon knowing that its drivers were removed, put into a 650 housing, then put back into the 660S2 and sent back to be resold (very likely, knowing Amazon, as new). And I think you yourself would pass on such a unit if you had the background knowledge before hitting "Purchase New".
This was one of those examples that solidified my resolve to never buy audio gear from Amazon.
Are you saying that you bought a new 660S2 and moved the drivers again, or did you send back that 660S2 with the 650 drivers after all?
As long as the unit wasn't damaged I wouldn't care. But then I understand the concept of a modular headphone. Returning in like new condition would mean the original drivers were in the headphone, so yes.