How to proceed?

@gdnrbob 

No doubt, but I've had these speakers for a long time, with other components and never had a sibilance problem.

If you cannot rise the bridge, you can lower river

If upgrades do no good, then downgrade and find older and cheaper one.

Price range for the DAC?

@czarivey 

Agreed -- I could always go back to the signal chain I had before sibilance and fatigue emerged. 

@mitch2 

To clarify, I'm not looking for particular dac recommendations so much as design characteristics/features associated with noise suppression to keep in mind. 

@stuartk You have a really nice system. That Aqua DAC seems really high quality and is R2R. I see good arguments in other threads that the DAC might be the problem. I find it hard to suspect the DAC, as it's well made and is R2R. Of course, if it's the DAC, that's a simple one for one switch.

If do wonder about the Hegel and (maybe) the room acoustics. Is there anyway you can borrow another amp to try out to see if that changes anything? If you tried an amp that was definitely on the warm side, you could really know about the Hegel. Looking at your room, I see there's not much about acoustics you can do that's easy. 

There is a nice discussion here -- some really good posts -- which indicate that room acoustics likely won’t solve it, though there is some debate about that. Some more discussion here, mentioning EQ and toe-in, among other ideas.