Krell's XD (Xtended Dynamics, Dimensionality & Detail) upgrade for the iBias amplifiers takes an already great sounding amplifier and raises its performance to the next level. XD is a classic example of Krell's continuous R&D efforts reaping benefits across multiple product lines. During the development of the K-300i Integrated Amplifier we discovered substantial sonic improvements by lowering the output impedance below traditional norms. When applying this technique to our existing line of amplifiers, the improvement was so substantial, it deserved its own, unique designation"XD.