do XLR to XLR IC's "insignificantly" reduce RF noise carried by RCA cables. resulting in blacker grounds and improved clarity and transparency??There's no universal answer, as it depends on many component, cable, system, and setup dependent variables. As I mentioned, though, more often than not I would expect other factors to be more significant, such as the quality of the single-ended to balanced converter circuitry in components that have unbalanced internal signal paths (i.e., that are not "fully balanced").
I operate the volume in the range between 12 o'clock to 2 o'clock.which may be greater than "upper 25% " of its range" as notedWhen I referred to the upper 25% I was thinking in terms of the physical position of a typical rotary-type volume control, which would have a range of adjustment from a minimum of perhaps 7 o'clock or so to a maximum of perhaps 5 o'clock or so. So 2 o'clock would not be in the upper 25%.
I haven't been able to find a spec or measurement of the gain, if any, that the Sphinx provides between its line-level inputs and its pre-outs. But given that according to Stereophile's measurements the overall gain of the Sphinx between its line-level inputs and its speaker outputs is 31 db, and the gain of your VK-200 is spec'd at 25 db, it seems conceivable to me that on some recordings you might find yourself turning the volume up to the 3 or 4 o'clock positions you indicated you would prefer to avoid. Personally, though, I would not consider that to be an issue. And in fact it might even be desirable, as in some designs high settings of the volume control may lessen any sonic side-effects that may be introduced by the volume control mechanism itself.
Best regards,
-- Al