OP what did you do in the end? I already know you don't like wide straps but I deliberately this morning played my heavy 5 string Warwick with a different strap,it's both less padded and skinnier than the normal one I use. Anyway after about 20 minutes I was really feeling it. Swapped back to the regular wide padded strap and the bass immediately "lost weight" ( I know it didn't really). Still have the heavy bass you mentioned?