The heaviest bass I have owned was a Washburn Scavenger which was just a whisker over 5kg. I have a massive soft spot for them - something about the symmetry of the body gelled, I liked the fact that it was a Precision sound without looking like a Precision, plus my one was in absolute museum condition. Still miss it, really, and on the rare occasions I have seen them on eBay I almost always stick them on my watch list while considering whether to go for it again. However common sense always prevails as there is no way could I wear one for more than 30 mins without paying for it next day. Plus I don't want any more 4 string basses.