LOVE me some Primus. Les Claypool is easily the best bass player in history to date.
I used to play bass in a band for about 4 years (never broke out but were pretty big in the underground scene). We used to cover My Name is Mud, shit....good times.
Play them there bass lines and tell me they aren't heavy... I don't mean heavy as in what you might consider heavy metal or whatever.... I am talking something different, maybe a different view. So, yes, heavy.