@antigrunge2 My experience is that anything that can be done to improve clocking between server & DAC is worth it. I can’t speak about MSB simply because I have never had a unit in to spend lots of time with but dCS definitely and Rockna and other also benefit. Remember MSB is R2R.
i am not going to say an external clock is universally essential but anytime clock synchronization can occur, jitter drops and sound quality approaches completely holographic. A great example of this is with Rockna and the I2S connection between a Wavedream and the Wavedream NET. The sun of those parts far exceeds what you would expect to get from those two components. When you look at the pin out, the R and L channels for the clocks are synchronized and grounded. Although it is not an external reference, the clean synchronization of the two has a huge impact.
There seem to be exceptions to this. Upscaling DACs like Chord (PCM) and Playback and EMM Labs (DSD) do not seem to directly benefit. Rob Watts is quite explicit that an external clock has no impact on Chord devices and both Playback and EMM work best with a fiber connection between streamer and DAC which has obvious benefits in terms of noise and clocking.
There are a myriad of other exceptions, but I am a firm believer that if external clocking can be added, it is likely a very worthy upgrade. USB especially benefits from this.