| If you don't have an owners manual, Google the amp make and model and you will be able to get one easily (or go directly to the manufacturer's website). You can also Google each control function to get an explanation.
Without a manual and not knowing the make and model of your amp, generally speaking, your last option is the best one. You should start with the pots in a neutral position. Depending on the type of pots you have, this may be at zero or at the 12 o'clock position. If your knobs are marked 0-10, it's probably the former. Play a little, adjust one of the pots, play some more. Keep adjusting one pot at a time and listen to the difference. Turning the a knob CW increases that sound characteristic and CCW decreases it.
Bass, Middle, and Treble controls are self explanatory. The Presence knob will boost or decrease the upper mid-range frequencies.
One caution: A 20W practice amp with only an 8" or 10" speaker may not give you a full dynamic representation of the sound variations that a more powerful amp/larger speaker combination will.
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