Good topic for me, have to jump in.
I have the NAD C658 preamp, the free dirac version (up to 500hz) is included. I'm only using that freebie so far, and I only toggle on when playing vinyl. I used the included microphone on the 15ft cord btw to setup.
Why? Because there's a major boost in gain, and without it my Starkrimson amp doesn't have enough gain (combined with phono amp, NAD impedance, unkown other factors).
BUT! the happy gain boost is ONLY when the 'analog bypass' is ON and the dirac profile I created is enabled. When streaming there's not much a gain difference, but there is a frequency/dirac difference (mainly bass is way flatter). I've thought about posting this oddity actually in a separate NAD / dirac post actually.
