How to proceed?

Borrow or buy a cheap low cost one to do the experiment and of different type than your actual one ...You will know if it is the dac synergy... Bad Synergy can exist also between high cost well designed components ...

@mahgister

Alright. I will start with a different DAC and see what occurs..

If all of your components and cables were chosen for their reputation of being highly detailed the presentation can lean too far in that direction. The leading edges of instruments and vocals are a little over emphasized and sibilance happens. A warmer cable in the mix might eliminate that and restore the balance. WireWorld and Cardas come to mind if you decide to try.The silver coated cable you're using might too much used throughout.

@jtcf

Resolution has never been a top priority. I don’t mean to be contentious but did you not read through the thread?

I wasn’t actually looking for more resolution. I’m much more a music lover who prizes emotional engagement. I was looking to correct the poorly-controlled bass I was getting in my previous system, which I found distracting and increasingly dissatisfying. Hegel amps are known for their high damping factor and the H390 has not only brought much better bass control but the speakers have come to come to life in a way that made me question whether I’d ever heard what they could do, before bringing in the Hegel

My IC and power cables are a mix of copper and silver-plated copper, chosen after finding all copper too dark and all silver too bright. My digital cable was described as having "a very liquid warmth that’s more gold than silver" in an Audio Bacon shoot-out among eleven contenders.

I’m not opposed to trying a different cable but I’d be surprised if swapping out a cable would have the same impact that results from removing the Aqua DAC from the system. The same coax BNC is used in both scenarios. When it connects transport to integrated’s internal DAC, fatigue disappears and sibilance is noticeably less exaggerated.

Hi @stuartk 

As you may recall, we have the same integrated amp. Having said that, when my very expensive Sony professional CDT died a few years ago, I ended up replacing it with a  Audiolab 6000CDT. An Audioquest Carbon digital to a Denafrips Pontus ll and XLR to the H390. And it sounds beautiful. I’m not familiar with your DAC, but were you running it with oversampling? Also, is your room treated with sound absorption? 

@stuartk 

Apologies if this was mentioned already, but I would consider trying different interconnects between the Aqua and the H390.  In your A/B tests, that is an additional change you are hearing (with and without the IC's).

Like your Aqua, I too have a R2R dac that uses vintage ladder chips known for natural sound with delicate and not bright highs.  I had sibilance issues of different degrees with a number of interconnects that were well regarded (Audience, Cerious, Audio Envy, Black Cat, others).

Eventually I found the right synergy with Zavfino Fusions IC's (silver and OCC copper).

Adding power conditioning to my system, (puritan psm) and different power cables (triode wire labs digital american) was another improvement in the high frequencies.

As to your original question on what dac designs to consider if you go that direction:  I'd stick with R2R, and look for designs that have intentional focus on a quality output stage and power implementation.