It's not about the signal being "clean", but about it being musical. There is room for huge scepticism about the tweaks that we are talking about here, but I have found that it is not about theory but about practice. The issue is what works. What matters is not how many clocks there are in a signal route, but what difference adding to the route makes. Sometimes it detracts from the quality of the sound that you are looking for. And the latter is dependent on what the rest of your system sounds like.
My own answers have been found by comparing the digitally-driven sound of a track in high resolution format with the same track on vinyl, comparing the various versions of the digital with the vinyl (the turntable being a pretty good one, with a good phono stage and leads). The digital combination that sounds most pleasing as compared with the vinyl is the one I go for. It may sound better (but not often unless I am using a DSD file: extracted from an SACD to a .dsf file), or it may not sound as good, but the key is what sounds best as compared with the vinyl. In that way, I have a constant reference and can be fairly sure that my ears are not playing tricks on me.
So, in answer to question 1: The X-SPDIF 2 is a digital to digital converter. It converts USB digital signals into co-axial and I2S digital signals. It is not like a W4S Recovery, which does something to the USB signal and outputs a further USB signal, but this time (in my experience) with a more relaxed and smooth sound.
As for question 2, I am not able to say what clock overrides any other: I just do not have the technical expertise to say. But as I indicate above, it is not about clocks, their number, and which one sort of wins. The issue for me is simply what works best in terms of giving me a sound that is as natural and musical as possible.
As for question 3, I have a Shunyata Venon internet cable and in the place where I use it, it is perfect. It's all about system synergy. I have found the Tellurium Q Black USB lead to be of good quality, i.e. sort of neutral and realistic, and I have also found the Nordost Blue Heaven to be the same but with a bit more bite. I have not tried any Tara Labs or Triode cables. If I were you, I would put in the Matrix Element H, use your existing Shunyata USB cable to go to the X-SPDIF 2, and then use a coaxial cable (RCA or AES/EBU) or, if you have an input for it, an HDMI cable carrying the I2S signal to your DAC. And then see how it all sounds. The Matrix units will clean up the sound tremendously, and the Shunyata cable between them will probably sound pretty good. You therefore may not need (or, perhaps I should say, want) to buy another cable.
