Per the Dreamline manual:
NOTICE – NEVER adjust the regulator above 160 BAR as damage to the system may occur and will void your warranty!
NOTICE – NEVER use the gun below the set regulator pressure as damage to the system may occur and will void your warranty!
They don't really specify a minimum, but I'm sure there's a point where it won't push the pellet out of the barrel. What that is, I have no clue. I have had mine as low as 100 BAR and it still shot fine, but never had it set lower than that.