Hi Pierre,
DISTORT tries to scale the transfer function to fit into the -1..1 range, regardless of what values you enter for the harmonics, so while the sound will be very distorted, it should never exceed 0dBFS

What did you hear when you tried to play with a positive dB value?
The reason I'm asking is that sometimes it is useful to specify a positive value so as to increase the amplitude above the default. I wouldn't want to disable this unless it's really causing harm.