The heaviest bass that I've ever owned and weighed was my late 90's Warwick Thumb NT 6. Beautiful instrument, all that wenge, bubinga and ebony, weighed 12 lbs. 2 oz. Worst neck dive of any bass I've ever had. At the same time I had a 2003 Thumb BO 6, and it weighed 11 lbs. The BO was much more comfortable to play, and sounded better. It was all ovangkol.
In 1980, I bought a brand new Music Man Stingray. Never weighed it, but it was probably at least 12 lbs. I was a young, reasonably fit man in those days, but that bass was painful to play for very long.