So I bought this NT-503 - mainly for the connectivity options that would allow my sons to plug in their laptops and bluetooth in their phones when they visit. I recently bought a 10mHz clock for this and I cannot believe the difference the external clock makes in the sound. Complete wall to wall sound, velvety, totally holographic. I don't understand why they wouldn't have built this technology in. It isn't that hard to get a low phase jitter 10mHz clock I would think. But for those of you with this unit - it is a different beast with the external clock. Holy moly.