I see Martin is following his usual path of denial
They are reasons that the power supply to a clock is also as important as the quality of the clock itself imho
For example the latest fad for fetmo clocks is quite something, uber low jitter sepc's the best yet etc. However EVERYTHING in the clock signal path way has an effect, personally I use low phase noise clocks for me they produce a far more natural sound, however the correct ultra quiet power supply rail to the clock is AS important to reaping these benefits.
Having fitted over 200 aftermarket clocks in the last 15 years, NO one has returned the units and decided it has made no difference what so ever.
So according to Martin's logic either they are over 200 people fooling themselves that it does make a different or like naim owners convincing themselves it does sound better than the rest of the audio world products
Or a lot of people are embarrassed but are too afraid to speak up?
Jitter is important in the context of the individual piece of equipment and how it is implemented, however the sum of the parts make the whole.
You can measure the before and after clock output frequency signals when you change a clock whether this relates directly to improved sound quality is as debatable as a class 'a' monster power amp is the epitome of quality audio play back.