The comparison was done carefully and systematically today with three very experienced audiophiles and myself. A cheap GPS disciplined clock (on the far right of the photo) was added as a reference.
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All listeners took turn to sit in the best position and ran through similar routine. First without any external 10M reference clock, we were free to choose our music, then GPSD clock was added, followed by Mutec SE-120, OP20 Limited and OP21A. Finally, the 10MHz clock was removed for the last piece.
We have similar impression. GPSD clock added more life to the music, more dynamic and more focus. Considering its price, around US$120, it is a best buy.
Mutec Ref 10 SE-120 is better than GPSD clock. It has a warmer and fuller sound. Bass is clearly improved. The result is pleasing and I can live with it forever. One listener particularly likes this sound. He is the owner of the Mutec SE-120 tested.
Cybershaft OP20 Limited2 has similar level of performance as Mutec but has a different character. OP20 sounds more lively and dynamic. Bass is not as strong as Mutec. In general, it is brighter than Mutec but has better focus and soundstage.
Cybershaft OP21A has the best of both worlds. It has the warm of Mutec and dynamic of OP20. The image and details are the best in the group. The difference is easily detected, not subtle. All of us agree it is the best performer.
After listening to OP21A for a while, the 10M reference was disconnected. The sound without it is plain and flat. It is like someone cover the speaker a thick cloth.
I can't speak for other digital systems, but if you use full stack of dCS Vivaldi system, the price of OP21A is around 10% of Vivaldi system and it improves the sound by 50%. You can do the maths. In fact, even OP20 and Mutec SE-120 enhance the sound significantly, the one who doesn't use a proper 10M reference clock on his Vivaldi system is not revealing the full potential of his gear..