For a transparent always-on boost set Gain low (2–3), Output above unity, and Treble around 5 (flat). The Klon is designed to keep your amp's character, so let the Output do the pushing and use the Gain sparingly.
Is a Klon a boost or an overdrive?
Both — at low Gain it is a clean/transparent boost that just makes your amp bigger, and as you raise the Gain it becomes a medium overdrive. It stays largely transparent throughout, unlike a mid-humped Tube Screamer.
Recommended starting points are community-researched estimates — every pedal, amp and guitar is different. Dial from here, then GuitarToneAdapt helps you match a target tone on your own rig. Pedal comparisons · Amp settings · All tones