Originally Posted by ProtegeManiac
Also they made the X1 much smaller, so likely the added parts that can allow USB DAC function might not even fit on the board.
Actually, the DAC function in the X3 is software-based; this was added in the firmware and a special USB driver for your PC/Mac earlier on without any hardware revisions. Same goes for the X5 (but this was done during product launch rather than an update).
Point being is that its overall structure is not strong enough (especially the CPU?) to support such software usage rather than missing any special parts that will almost immediately give it USB DAC functionality.