Hi Perfusionist, I don't know if this will help, but....
Regarding power leg requirements, I suggest reaching out to LampiNA, as he is very familiar with both FTA and Lampizator products. You may have to review the SoTM requirements, likewise, for the complete picture however. The power leg in the USB cable
may be necessary for certain electronics to initialize, i.e., handshake and recognition of each other, when powering up. I believe that is the case for Innuos products. Both of my FTA USB cables are 'with' the power leg in my current arrangement: Innuos -> Innuos -> DAC. Incidentally, I have not tried a USB cable w/o a power leg.
Regarding mixing USB's, I have two small comments/recommendations originating from others, which may (or may not) be helpful for you. First, Innuos (and others) suggests using 'the same' USB cable before (dirty side) and after (clean side) the Phoenix USB Reclocker as a starting point to establish a valid aural baseline - to get a handle on what is going on - sound wise. For you, that may not be possible, assuming you only change 1 USB at a time. The second suggestion given to me was: if you use different USB cables, consider putting the better one Upstream (on the dirty side) feeding the reclocker. Some feel this would be the better approach/allocation. Unfortunately, I am unfamiliar with your current USB cables, so you will have to be the judge
. Of course, the results are completely subjective, and only you can make that determination. That said, your plan to start changing only 1 variable at a time makes sense, and would appeal to "scientific method" approach, for re-baselining your experience.
Hope those help.