You are quite right here. Mentor menuets sound quite bright in the setup. I love the presentation of the DAC especially in acoustic music. I dont have any issue with music itself but vocals (especially male vocals) sometimes sound harsh. I am not sure its because of resolution of the setup or source issues or because of speakers.
I have (had) similarly bright speakers in KEF. I tried many different amps but the brightness was always there. I'm all for detail and clinical accuracy, and prefer that to warm speakers that are more musical but lose the details, but I still like detail without extra brightness.
Anyways the brightness disappeared when I used a Raspberry Pi for the first time in my setup. Never ever thought that a change in source could have so much of an effect, but it did. That's one reason why I stress source separation and galvanic isolation, it made a huge difference in my setup.
I'd go as far to say don't use USB unless you have to... as in high res music over 24/96. Whatever the limitations of Toslink, the benefits and gains in SQ it offers with galvanic isolation cannot be denied and may be even worth the trade off.
Also, they are huge tweaks on the cheap. $35 for the CCA + $35 for the Pi, that's $70 in tweaks that will be better than $700 tweaks in most instances like $700 spent more on an amp, speakers, or even cables. You may or may not have similar results, but at $35 it hardly costs anything and if nothing you can always use it to stream radio. If it works then at least it will point you in the right direction of eliminating noise floor from your system.
That said, its only one end of the spectrum on improving the SQ in your existing setup.
Amps are often neglected, but going from a $800 to a $1800 amp made my speakers sing unlike anything. They raised the bar on my speakers like 3x.
Then I plugged a Yggy into my system and man it was a revelation. Unfortunately I could only have it for 2 days, so it was not enough time to listen to all of my music "for the first time" again.
I'd venture to say the tweaking is never ending... but thankfully we can have most of them without costing an arm or a leg.