what guitar have you got?
i actually find most amps only have one or two sweet spots in terms of sound, then i just vary it on guitar switching to different pickups, rolling back volume and adjusting tone.
anyway for reggae, id stick to clean channel, switch to back/middle pickup on guitar. on marshall amps i always heavily roll down the treble as find them harsh and tinny for clean sounds. something like bass 6 mid 7 treble 4. i wouldnt have any of the fx on for reggae either. just a nice clean sound, no reverb
hope that helps!