#4
Well assuming that it's a single channel amp (most multi channel amps have a dedicated clean channel), most guitarists would set the amp for a crunchy tone and roll back on the volume knob for cleans, exactly as you described.
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#6
a lot of ways, if the amp is two channels a clean and an OD then a footswitch usualy comes with the amp.

if there is only one channel set it to the clean you want and then you can buy a distortion pedal to kick on and off.

depending on how heavy the distortion is set too, you can also roll back your volume knobs to clean it up a bit but this will cut volume as well, a similar effect to switching your distortion pedal off and turn the volume knob down