What amplifier (brand, model) and what acoustic guitar are you using?

I’m assuming that you’ve plugged an acoustic guitar into your amp, and that you’re not satisfied with its tone.  Usually if the acoustic guitar has a cheap piezo pickup, it may not sound good (harsh/shrill/piercing sound), especially if the solid state amp is loud and clipping.

Generally, it’s advisable to use the clean channel volume set low (below 12 o’clock), then proceed with the treble, mid and bass knobs at 12 o’clock, and adjust accordingly to your taste.