How to proceed?

@curiousjim

Take a look at my virtual system page and you will see it is not at all optimal for audio. There is a wall of glass on one side, a tiled hearth, glass topped coffee table, etc. WAF disallows any panels. Nevertheless, my previous system (same speakers + dac, with Wells Audio Majestic integrated and Simaudio transport) was not at all fatiguing and there was not a trace of sibilance. The bass was poorly defined/controlled, which eventually drove me nuts and was the primary motivation for acquiring an integrated known for bass control. The Hegel is, needless to say, far more resolving then the tubey-souding Wells and this aspect, coupled with the Jay’s, which similarly delivers more detail than the Simaudio, may be too much of a good thing. DAC is NOS.

@veroguy

Thanks for sharing strategies you’ve found to be successful. You are astute in recognizing the ICs are also absent from the scenario with the least sibilance and no fatigue. I’ve heard good things about Zavfino. I don’t recall whether they have a return policy. Ditto Triode Labs. I know Jaguar does.

One dac that interests me is Border Patrol, which is very much designed with particular attention to power implementation.

I don't believe you mentioned your choice of digital cable.

@stuartk ...my previous system (same speakers + dac, with Wells Audio Majestic integrated and Simaudio transport) was not at all fatiguing and there was not a trace of sibilance.

...One dac that interests me is Border Patrol, which is very much designed with particular attention to power implementation.

This is a key indicator to me. Since the new integrated and new transport was introduced, the issues you describe started to occur. Also, re-check your path of any new "silver" based cables (power, interconnect) you introduced around the same time. If you have stock power cables lying around, try all together at once.

btw, I own the new Border Patrol SE-I dac, while I agree its smooth and non fatiguing, if I may suggest, I don’t think the dac change alone is going to resolve all of your noted issues. I kind of suspect its a combination of things going on now. Imo, as a test, I’d borrow/swap the amp and transport and [all] cables back out with stock ones all at once. Then go in reverse putting new pieces back in play one-by-one to see if it surfaces again. Worth trying to help understand if you can narrow down a specific culprit (or combination of things) from there. Best of Luck.

@decooney

Thanks for your suggestions. They make sense. Unfortunately, I do not have any unused gear lying around and don’t know anyone who would loan me the gear.

One exception-- I tried my old tubed Joida CDP as a transport and the sibilance was worse.

@stuartk my understanding is you have tried different cables between dac and amp and that didn’t solve the issue. Those were DH Labs cables. 
In my experience DH Labs cables are brutally honest and lean slightly on a brighter side of neutral. 
I will give you an example from my most recent cable swap/upgrade experience…

I was using Acoustic Zen Absolute Copper XLR between the Bricasti M3 DAC and XP22 pre, as well as between the XP22 and X260.8 monos. 
The AZ Absolute Copper were a bit too warn for my system with the latest upgrades. So I called The Cable Co and tried the following: Audience AU24SX, Analysis Plus Silver Apex, Kimber KS1126. I had on hand AZ Silver Ref II and the Nordost Tyr 1 and Tyr 2 that my friend loaned me. 
There was a significant increase in sibilance with the AP Silver Apex. A bit less with Audience AU24SX but still present, there with AZ Silver Reg but no sibilance with Kimber and Nordost Tyr 2. I never experienced sibilance in this system with the AZ Absolute Coppers. There was always a super smooth midrange with delicate treble but the bass was too plump with my Wilsons and the Pass monos. I struck gold with Nordost Tyr 2 XLRs. My speaker cables are Audience AU24SX 

So the moral of the story is - you still have not done due diligence with the cables. 

@audphile1 

Thanks for your suggestions.

I have used The Cable Co. in the past and felt very positive about them but my last experience was so negative, I'm not sure I'd want to go back. 

I had A. Zen Silver Reference and they worked great in my previous system but were too bright once I acquired the Hegel and Jay's. So I tried the Absolute Copper but they were extremely dark sounding, top to bottom. 

I'd be very happy if my problem could be solved via cables.