I don't pretend to be the truth, but I'll tell you something else. Buying NAD amplifiers with a slot for installing the MDC BluOS module without the module itself is money in the wind!)))
Why I think so, again from my own experience. No external streamer connected via a coaxial digital input sounds better than their own development - the BluOS module installed into amplifier. Because the PCM signal go directly through system port. Specifically, in our case with the C399, we have a module with a two-way communication with an advanced DSP installed on board, and all incoming signals from any other inputs can be processed too. In fact, a modern advanced digital equalizer is installed on the new BluOS-D board. For uninitiated users, this is all reduced to automation, and for those who are advanced, they buy an extended version of Dirac and draw themselves any frequency response. Buying NAD C399 without BluOS-D module - it is a crime!