Looking at your system and the comments, I assume that the sibilance was not an issue with the Wells and Moon setup? The Aqua is a pretty nice unit, I would be surprised that it is the source of the problem. Possibly it is making it more apparent, and the Hegel DAC smooths it over a bit. Playing Devil's Advocate, have you tried replacing the source (Jay's) with something else to see if the problem persists? I also thought to suggest bypassing the Hegel preamp, but looking at the back side it does not look like that can be done with this integrated.
How to proceed?
Removing my Aqua La Voce dac from my system has eliminated fatigue and reduced but not fully eliminated sibilance. Running my Jay’s CDT2MKIII into my Hegel H390’s onboard dac is definitely a more relaxed presentation but tonally leaner than I prefer. Some have suggested I swap out the Hegel, but first I’d like to try another dac.
What I’m unclear about is how to go about choosing another dac that will not duplicate the same drawbacks I’ve experienced with the Aqua. Are there details in the design or specs that can guide me in this regard? I'm unsure how to proceed.