Treble boost adds air and detail to a dull recording, but it's also the easiest band to overdo.
Start with 2–3dB at 8kHz and listen for sparkle rather than sharpness. If cymbals or S sounds start to feel piercing, that's the sign you've crossed from "bright" into "harsh," and it's time to dial back.
Human hearing is especially sensitive to upper frequencies, so even a small treble boost can make a track feel noticeably louder without any change to the master volume — worth remembering if a boosted track suddenly feels more fatiguing to listen to.