Try using a dedicated endpoint to connect to the DAC. Roons recommendation is to seperate the core server part from the music playback so it’s all connected over ethernet as the core is a very busy piece of software and this may or may not affect the sound quality as computers are noisy things electrically which can traverse down usb to DAC. You can create one with another computer or a raspberry pi SBC and install the Roon bridge software on it, latter are the most popular option.
I am surprised at this statement?. I have an Naim DAC V1 and it is connected to an iMac 2019 and use a pair of DT770’s when listening to HI-Res ( 24/44 and above ) files only because they have a dark sounding nature! and higher resolution helps them open up slightly, and for everything else I use a pair of Sennheiser HD650’s. Try other headphones if you can as the Naim’s headphone output is very efficient and I would not normally go above 50% volume. It’s surprising how different headphones sound with various bit rates.
You haven’t by any chance changed any of the parametric eq have you?. If not try connecting another device to the DAC if possible and also connect your headphones direct to the MacBook Air to eliminate possible causes for the bad sound you describe, not ideal but may give you a clue as to what is going on. Good Luck.