stoker will likely be worse, the bigger the lump the cleaner and smoother the burn with bit coal. Some bit coals make more smoke and soot than others; try a different supplier of lump bituminous coal, maybe you will find better stuff. you could try building a hot fire (leave the ashpan open with a bed of red coals) to blow/burn the soot out every day or every other day, then you won't have to clean it as much. Either way, a 6" stack is too small. if you had an 8" or larger stack size cleaning wouldn't be an issue, the soot would periodically fall off and never build up to be a problem. 6" just clogs right up with some coals.