As a first step in trying the affect of 'tuning' changes, try the different existing sound signatures available in the Settings menu, if you've not already done so. There are audible differences and you may find one that suits your audio set up.
My hearing is no longer at optimum (several years older than you...) and I was unable to hear any differences with the changes suggested by Archimago, although they should result in a less stressed Pi board.
I have been able to hear improvements after experimenting with the PSU. My first step was putting a ferrite sleeve around the cable from the official Pi wall wart PSU I originally used. Although I'd not specifically noticed the noise before, this cut out all the hi-frequency mush from the PSU and gave a 'blacker' background. (Something like this
https://uk.rs-online.com/web/p/ferrite-sleeves/7740639/ )
Encouraged by this and other posts I'd read, I built a low noise linear PSU (board from Ebay + torroidal transformer). On fast percussive music in particular, there is greater clarity. A further improvement was to take the power feed for the Pi/DAC combination to the DAC via the GPIO pins, rather than via the Pi power input. The sound stage widened when I did this. (It may simply be that the DAC board is then getting a proper 5v supply, instead of 4.9v or so when it is powered via the Pi).
After many years of experimenting with Hi Fi I am aware than many tweaks and improvements are subjective and can be the result of nothing more than a very slight increase in volume or tone. It's still very satisfying trying to make improvements.
Ian
Pi2 + IQaudIO Pi-Dac+
Linn Majik Amp. Epos ES12i.
