I continue to believe that you have a jitter problem. This is related to either the quality of your clocks or the cables admitting RFI/EMI interference. Both AES and BNC are sending the clock signal to the DAC whereas an asynchronous USB connection would slave the transport to the DAC‘s clock. If you have opticsl connection between the two that would be the cheapest way to check whether it‘s interference or clocking.
Sibilance # 2 or "Boy, do I feel like an idiot"
I decided to try what @ghdprentice had suggested-- I took the Jay's out and tried both the Sim, then a Jolida JD100 CDP (with its DAC bypassed) and... I heard the same sibilance as with the Jay's !
I'm baffled-- I have no explanation and I'm sorry for wasting everyone's time.
Before I got the Jay's (using the Sim transport.with the Hegel) I wasn't aware of any sibilance,
My wife, a decidedly non-obsessive but wonderfully perceptive bystander, is dead certain there was no sibilance before I acquired the Hegel. At that point, I used the Sim transport with a Wells Majestic integrated.
Should I now conclude the issue is with the Hegel ?
Not that there is anything malfunctioning, but that there is a synergy issue?